CSS: Remove the constants MHU_SECURE_BASE/SIZE

For CSS based platforms, the constants MHU_SECURE_BASE and
MHU_SECURE_SIZE used to define the extents of the Trusted Mailboxes.
As such, they were misnamed because the mailboxes are completely
unrelated to the MHU hardware.

This patch removes the MHU_SECURE_BASE and MHU_SECURE_SIZE #defines.
The address of the Trusted Mailboxes is now relative to the base of
the Trusted SRAM.

This patch also introduces a new constant, SCP_COM_SHARED_MEM_BASE,
which is the address of the first memory region used for communication
between AP and SCP. This is used by the BOM and SCPI protocols.

Change-Id: Ib200f057b19816bf05e834d111271c3ea777291f