ARC: NSIM: switch to ns16550 UART model

Switch nSIM from custom ARC UART to ns16550 model. That will
allow us to use zephyr images built for nSIM on other platforms
like HAPS, QEMU, etc...

This patch do:
 * switch nSIM board to ns16550 UART usage
 * change nSIM simulator configuration to use ns16550 UART model
 * drop checks for CONFIG_UART_NSIM in ARC code
 * update nSIM documentation

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
13 files changed