ArmPlatformPkg: allow patchable PCD for FD base address

ArmPlatformPkg: allow patchable PCD for FD base address

This moves the reference to gArmTokenSpaceGuid.PcdFdBaseAddress
from the [FixedPcd] to the [Pcd] section in the INF file of
PrePiArmPlatformGlobalVariableLib so that its users may choose
to use a patchable PCD instead.

Contributed-under: TianoCore Contribution Agreement 1.0
Reviewed-by: Olivier Martin <olivier.martin@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16955 6f19259b-4bc3-4df7-8a09-765794883524