lib/utils: irqchip: Allow M-mode IMSIC files to be sparse and closer to S-mode

The current sanity check expect M-mode IMSIC files to be in a separate
region which is only true if a platform supports PMP. Any platforms without
PMP support may keep M-mode IMSIC file along with S-mode/VS-mode files.

Signed-off-by: Atish Patra <atishp@rivosinc.com>