serial/serial_arc - add driver for ARC UART

Driver for non-standard on-chip UART, instantiated in the ARC (Synopsys)
FPGA Boards such as ARCAngel4/ML50x

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>

Cc: Mischa Jonker <mjonker@synopsys.com>
Cc: Francois Bedard <fbedard@synopsys.com>
Cc: Tom Rini <trini@ti.com>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Stefano Babic <sbabic@denx.de>