lib: utils/serial: Ensure proper allocation of PMP entries for uart8250

The added memory region should start from the base address.
Otherwise, the range will be shifted by reg_offset and not
able to merge consecutive NAPOT regions in the root domain,
resulting in wasted PMP entries.

Fixes: e8bc1621

 ("lib: utils/serial: Add shared regions for
serial drivers")

Signed-off-by: Yu Chien Peter Lin <peterlin@andestech.com>
Reviewed-by: Anup Patel <anup@brainfault.org>