serial: enable 64-bytes FIFO for UART 16750 in uart_ns16550 driver

UART 16750 is basically a UART 16550 with a 64-bytes FIFO.
The 64-bytes FIFO can be enabled via register FCR.

Account for it in the uart_ns16550 driver whenever the
CONFIG_UART_NS16750 variable is set.

Change-Id: I2342b28a41d03c96410bbfbe57e4b5a4e335731d
Signed-off-by: Jean-Paul Etienne <fractalclone@gmail.com>