ArmPlatformPkg: Allow PcdFirmwareVersionString to be a dynamic PCD

ArmPlatformPkg: Allow PcdFirmwareVersionString to be a dynamic PCD

PcdFirmwareVersionString is defined in MdeModulePkg to be
either fixed or dynamic, but is restricted in ArmPlatformPkg
drivers to FixedPcd. Changed to remove the FixedPcd
restrictions to allow platforms to chose the correct type
in their DSC files.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Samer El-Haj-Mahmoud <samer.el-haj-mahmoud@hp.com>
Reviewed-by: Olivier Martin <olivier.martin@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17833 6f19259b-4bc3-4df7-8a09-765794883524