module/scmi_system_power: Handle unsupported SCMI message

In the SCMI handler table, unsupported messages are indicated by leaving the
corresponding function pointer unassigned. However, there is no validation
check on entry which leads to potential null pointer dereference while handling
message. This change explicitly initializes the handler table and validates
entry in runtime to determine whether a given message is supported.

This change addresses the security vulnerability (CVE-2024-11864).

Signed-off-by: Wei-Chia Su <Wei-Chia.Su@arm.com>