lib: utils: Improve fdt_parse_uart8250() API

The information parsed by fdt_parse_uart8250() API is not complete.
We need to parse reg-shift and reg-io-width for UART 8520 as well.

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