Fix BL2 memory map when OP-TEE is the Secure Payload

The commit 3eb2d672 optimizes the memory map for BL2 when TSP
is not present. But this also broke OP-TEE as it was reusing
the TSP mapping. This patch fixes this problem by adding a
separate mapping for OP-TEE in the BL2 memory map table.

Change-Id: I130a2ea552b7b62d8478081feb1f4ddf5292a118
Signed-off-by: Soby Mathew <soby.mathew@arm.com>