ArmPkg/BdsLib: Fixed manipulation of the Memory Map returned by GetMemoryMap()

ArmPkg/BdsLib: Fixed manipulation of the Memory Map returned by GetMemoryMap()

The UEFI specification mandates that software uses the DescriptorSize returned
by the GetMemoryMap() function to find the start of each EFI_MEMORY_DESCRIPTOR
in the MemoryMap array. This allows for future expansion of the EFI_MEMORY_DESCRIPTOR.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Olivier Martin <olivier.martin@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14447 6f19259b-4bc3-4df7-8a09-765794883524