lib: utils: serial: Initial commit of xlnx-uartlite

Initial commit of the xlnx-uartlite device and FDT support. This was
tested by running OpenSBI on a modified QEMU virt machine using the
xlnx-uartlite for serial.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup@brainfault.org>