RSE: Fix MHU shutdown channel id and payload for RD-V3

Fix MHU shutdown channel id for RD-V3 platform. This patch also consider
the transport message in SCMI format, and hence use the second element
of payload to get the system powerdown mode.

This patch is a stopgap solution until SCMI support is enabled in tf-m.

Change-Id: I73f1c7c3f8940841b6865539ab0f6e59afd9f78a
Signed-off-by: Pranav Madhu <pranav.madhu@arm.com>