lib: utils/irqchip: Add shared MMIO region for PLIC in root domain

On platforms with Smepmp, the MMIO regions accessed by M-mode need
to be explicitly marked with M-mode only read/write or shared (both
(M-mode and S-mode) read/write permission.

If the above is not done then runtime PLIC access from M-mode on
platforms with Smepmp will result in access fault when further
results in CPU hotplug not working.

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