Microchip: MEC172x: Add UART driver

Microchip XEC has been using the standard NS16550 driver.
Using the standard NS16550 driver requires extra HW programming
for XEC UART in board level and did not support XEC GIRQ interrupt
programming. We add an XEC specific driver and remove UART specific
register programming from the board level and implement interrupt
support. Also, by implementing a SoC specific driver we can add
driver PM in the future.

Signed-off-by: Scott Worley <scott.worley@microchip.com>