serial: 8250_dw: add fractional divisor support

For Synopsys DesignWare 8250 uart which version >= 4.00a, there's a valid divisor latch fraction register. The fractional divisor width is 4bits ~ 6bits.

patch1 introduces necessary hooks to 8250 core.
patch2 exports serial8250_do_set_divisor()
patch3 implements the fractional divisor support for Synopsys DW 8250.

Since v5:
- Add Andy's Reviewed-by tag to all patches.

Since v4:
- use tmp to save baud * 16 rather than calculate every time in dw8250_get_divisor()

Since v3:
- simplify the dw8250_get_divisor() implementation again.

Since v2:
- rebase to tty-next branch, since I need one patch from Andy which is in tty-next
- drop the patch "serial: 8250: let serial8250_get_divisor() get uart_port * as param" since it's in tty-next now.
- add a new patch to export serial8250_do_set_divisor(), and reuse it to complete dw8250_set_divisor().
- remove DW 8250 version check, since the DLF register always exists and if fractional divisor isn't supported, the register read as 0
- add comments to explain how dw8250_get_divisor() get quot and frac.
- the frac calcuation is simplified with well implemented GENMASK
- Add Andy's Reviewed-by tag to patch1.

Since v1:
- add an extra patch to let serial8250_get_divisor() get uart_port * as param
- take Andy's suggestions to "integrates hooks in the same way like it's done for the rest of 8250 ones". Many thanks to Andy.

Jisheng Zhang (3):
serial: 8250: introduce get_divisor() and set_divisor() hook
serial: 8250: export serial8250_do_set_divisor()
serial: 8250_dw: add fractional divisor support

 drivers/tty/serial/8250/8250_core.c |  4 +++
 drivers/tty/serial/8250/8250_dw.c   | 45 +++++++++++++++++++++++++++++
 drivers/tty/serial/8250/8250_port.c | 30 +++++++++++++++----
 include/linux/serial_8250.h         |  3 ++
 include/linux/serial_core.h         |  7 +++++
 5 files changed, 84 insertions(+), 5 deletions(-)

2.18.0