morello: fix broken SCP-MCP SCMI communication

This patch has the following changes for MCP-SCP SCMI comms:
1. Because of an issue with unaligned accesses with MHU SRAM, move
MCP-SCP MHU mailbox memory region from SRAM to AP Non-trusted RAM.

2. As MCP needs to wait for SCP to initialize certain clocks before
it can access the system access port, MCP will poll MCP2SCP
Secure MHU RAM for a handshake pattern before continuing the boot.
SCP writes this handshake pattern before initializing AP core.

3. Updated policies in SCP(config_transport.c) and MCP(config_smt.c)
for MCP-SCP SCMI service.

4. As the shared mailbox address is in AP Non-trusted RAM, changing
from secure to non-secure channel. Also made relevant changes in
config_mhu.c for SCP and MCP.

Signed-off-by: sahil <sahil@arm.com>
Change-Id: I49b95aba8920cbd265603abd2db7a6b69c4baa4f