feat: add build options for memory protect

In preparation to adding a platform module to implement memory
protection, add "memory_protect" field to platforms.

In following patches the referred configuration is used to link
the platform specific module.

Change-Id: I07ec4a138868429dd2777a0dc08f5548c065c055
Signed-off-by: J-Alves <joao.alves@arm.com>
diff --git a/build/toolchain/platform.gni b/build/toolchain/platform.gni
index 4c0c7a0..6d99c1d 100644
--- a/build/toolchain/platform.gni
+++ b/build/toolchain/platform.gni
@@ -21,6 +21,10 @@
   # IOMMU driver to be used for the platform, specified as build target.
   plat_iommu = ""
 
+  # Abstract the calls to platform memory protection functions.
+  # E.g. FVP leverages RME if present to change PAS (S or NS) of given address.
+  plat_memory_protect = ""
+
   # The number of pages to allocate for the hypervisor heap.
   plat_heap_pages = 0