lib: utils: Add fdt_parse_uart8250_node() function

We add fdt_parse_uart8250_node() function which will allow us
to parse a particular DT node as UART 8250 node. This will be
useful in parsing the node pointed by stdout-path.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>