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

Signed-off-by: Balaji Gontumukkala <balaji.gontumukkala@arm.com>