lib: utils/fdt: Remove redundant code

uart->reg_offset and uart->reg_io_width are only used on uart8250 and
not required on other platforms. Remove for sifive and gaisler.

Signed-off-by: Xiang W <wxjstz@126.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>