drivers/tty/serial: add LiteUART driver

This commit adds driver for the FPGA-based LiteUART serial controller
from LiteX SoC builder.

The current implementation supports LiteUART configured
for 32 bit data width and 8 bit CSR bus width.

It does not support IRQ.

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>
Signed-off-by: Mateusz Holenko <mholenko@antmicro.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stafford Horne <shorne@gmail.com>