ArmPlatformPkg: Change the memory model for the ARM Platform components

ArmPlatformPkg: Change the memory model for the ARM Platform components

In the former memory model, the UEFI firmware was expected to be located
at the top of the system memory. Stacks & Pi memory regions were set below
the firmware.
On some platform, the UEFI firmware could be shadowed by the ROM firmware
(case of the BeagleBoard) and in some cases the firmware is copied at the
beginning of the system memory.

With this new memory model, stack and Pi/DXE memory regions are set at the
top of the system memory wherever the UEFI firmware is located in the memory
map.
Because DXE core does not support shadowed firmwares, the system memory covered
by the UEFI firmware is marked as 'Non Present' to avoid to be overlapped by
DXE allocations.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@11992 6f19259b-4bc3-4df7-8a09-765794883524