SPM: Move RPC message check definition into spm.h

It belongs to SPM's scope to check if the connection message is
for RPC request.

Also, the check function is renamed to "tfm_spm_is_rpc_msg()".

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: I305d12ce0ee56d6c84b9618e7fa1ff093bd83f5b
5 files changed