drivers: serial: add virtual uart driver for nsim

In nsim, 'nsim_mem-dev=uart0,base=0xf0000000,irq=24'
is used to enable a virtual uart.

This uart is only used to output now, so only poll output
is implemented.

If required, input and interrupt driven are possible.

Signed-off-by: Wayne Ren <wei.ren@synopsys.com>