ArmPkg: ArmArchTimerLib: conditionally rebase to actual timer frequency

ArmPkg: ArmArchTimerLib: conditionally rebase to actual timer frequency

Allow a platform to set PcdArmArchTimerFreqInHz to zero, and consider it a
request to use the actual timer frequency.

No global variable is introduced, so that the library can be used in PEIMs
that execute from flash.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Olivier Martin <Olivier.martin@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16691 6f19259b-4bc3-4df7-8a09-765794883524