lib: utils: Add LiteX UART support

Add support for the UART provided by the LiteX SoC
framework (https://github.com/enjoy-digital/litex

),
based on its FDT info (described in the Linux tree at
Documentation/devicetree/bindings/serial/litex,liteuart.yaml).

Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>