lib: utils/serial: Add shared regions for serial drivers

The serial driver regions used by OpenSBI should be marked as a shared
read-write regions between M-mode and SU-mode as those are accessed
by earlycon and the corresponding tty serial drivers running in 'S' mode.
When the smepmp extension is enabled, PMP entries for these shared regions
will get programmed.

Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>
Signed-off-by: Anup Patel <apatel@ventanamicro.com>