This is the initial commit of the Ibex UART device. Serial TX is
working, while RX has been implemeneted but untested.

This is based on the documentation from:
https://docs.opentitan.org/hw/ip/uart/doc/Signed-off-by: NAlistair Francis <alistair.francis@wdc.com>
Reviewed-by: LIU Zhiwei<zhiwei_liu@c-sky.com>