lib: utils: Add fdt_parse_sifive_uart_node() function

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

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