gpio: 74x164: seed the chain from DT at probe time

This v2 reuses the existing lines-initial-states bitmask, as suggested by Linus Walleij, instead of inventing a new property. That sidesteps both the v1 review concerns at once: there is no vendor-specific property to argue about, and the bit-N-equals-line-N convention is already documented for nxp,pcf8575.

Background (carried over from v1):
The 74HC595 / 74LVC594 family is push-pull output only. There is no read-back path, no defined power-on state for the parallel outputs, and __gen_74x164_write_config() always shifts the full chain on every transaction. The driver therefore publishes whatever sits in chip->buffer on the very first ->set() call, and today that buffer is fresh from kzalloc(), so all outputs come up low. On boards with active- low indicators or other signals whose initial level matters, the 0x00 publish glitches the chain before user space (or gpio-leds default-state walking) gets a chance to run.

gpio-hog is not a substitute here: each hog is applied sequentially and forces an intermediate full-chain write per line, and on this board gpio-leds already claims every output, leaving no free lines to hog.

Approach in v2:
Patch 1 documents the existing lines-initial-states property under the fairchild,74hc595 binding, with the small wording change that on this output-only family bit=0 drives the line low and bit=1 drives it high.

Patch 2 wires the driver to read that bitmask into chip->buffer before the first __gen_74x164_write_config(), so the chain comes up in the configured pattern atomically on the first SPI transaction. Property absence keeps the existing zeroing behaviour.

The bitmask covers up to 32 lines (four cascaded chips), which fits all existing in-tree users and the typical 1-4 chip cascades. If a longer chain ever needs seeding, the property can be extended to a uint32-array without breaking the bit-N-equals-line-N convention.

Tested on a 4-chip 74HC595 chain (32 outputs, active-low LEDs); the 0x00 glitch on first write goes away with lines-initial-states set to the desired idle pattern, and absence of the property leaves behaviour unchanged.

Changes since v1
[https://lore.kernel.org/linux-gpio/[email protected]/]:
- Drop the new 'registers-default' u8-array property; reuse the existing 'lines-initial-states' uint32 bitmask instead, as suggested by Linus Walleij. This also addresses Krzysztof Kozlowski's concern about adding a generic, non-vendor-prefixed property to a vendor-specific binding (the property is already documented for nxp,pcf8575).
- Driver now reads a single u32 and maps bit N to GPIO line N, matching the nxp,pcf8575 convention. For this output-only device the polarity is the natural one: bit=0 drives the line low, bit=1 drives it high.
- Binding example updated accordingly.

Chanhong Jung (2):
dt-bindings: gpio: fairchild,74hc595: add lines-initial-states
property
gpio: 74x164: support lines-initial-states for boot-time output state

 .../bindings/gpio/fairchild,74hc595.yaml        | 13 +++++++++++++
 drivers/gpio/gpio-74x164.c                      | 17 ++++++++++++++++-
 2 files changed, 29 insertions(+), 1 deletion(-)

2.34.1