gpiolib updates

Following this are several patches updating the current gpio implemenentation framework. Because one of those changes involves creating a new drivers/gpio directory, the patches are a bit simpler if two existing patches are first removed from the MM tree:

mcp23s08-spi-gpio-expander.patch mcp23s08-spi-gpio-expander-checkpatch-fixes.patch

The patches in this series are:

- Adding a gpio_desc[] layer, making it easier to mix in gpio_chips with different numbers of GPIOs without any large holes in the number sequence. When those holes are also reflected in IRQ number sequences, they cost a lot of memory. (Based on work from Eric Miao.)

- More diagnostics with CONFIG_DEBUG_GPIO. Handy when bringing up a new board, needless later. (Various platforms had similar diagnostics.)

- A bit of implementor-oriented documentation for this gpiolib infrastructure.

- Create a new drivers/gpio directory for expanders and (eventually) other code. (Strongly encouraged by Jean Delvare, to help shrink drivers/i2c/chips.)

- New-style I2C driver for pcf8574/pcf8575/compatible GPIO expanders ... these are pretty widely used. (This was previously posted as patch/rfc.)

- Relocated mcp23s08 driver. This includes two minor functional changes, to handle the gpio_desc changes and modified driver init sequence, and it merges the minor checkpatch.pl update above.

There's an updated pca9539 driver in the works too; it should land in the drivers/i2c directory.

I'm thinking there should be a non-functional change to the gpiolib code: move it from lib to drivers/gpio.

My question is whether that's better done by replacing the current patches with one new patch, or by a patch deleting the current lib/gpiolib.c and adding a new drivers/gpio/gpiolib.c ... I think the former would make more sense to anyone looking at GIT history.

- Dave