lib: sbi_trap: Allow M-mode to M-mode ECALLs

We should allow M-mode to M-mode ECALLs because:
1. No other mode can handle M-mode ECALLs
2. In future, we can allow M-mode baremetal application (not
   linked to OpenSBI) running independently and cooperatively
   without corrupting OpenSBI M-mode state (CSRs and memory).

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>