serial:xilinx_uartps: Add OF alias support for PS uart

Check if the PS uart is in the alisa list, if yes, use this id. Else
use the default detection.

Signed-off-by: Jason Wu <jason.wu@petalogix.com>

tty: serial: Use clock-frequency property instead of pure clock

clock-frequency is widely used in all FD drivers.

Signed-off-by: Michal Simek <monstr@monstr.eu>