lib: utils/serial: support 'reg-offset' property

reg-offset property is used for offset to apply to the mapbase
from the start of the registers in 8250 UART. In Linux kernel,
it has been handled in 8250 UART driver.

dt-bindings:
<linux>/Documentation/devicetree/bindings/serial/8250.yaml

Signed-off-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>