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>