ArmPkg: add reenable hook to ArmGenericTimerCounterLib

ArmPkg: add reenable hook to ArmGenericTimerCounterLib

In preparation of selectively reinstating the timer enable quirk for Xen
that we removed in commit 411a373ed642 ("ArmPkg/TimerDxe: remove workaround
for KVM timer handling"), add a ArmGenericTimerReenableTimer() library
function to ArmGenericTimerCounterLib that we will populate for Xen only.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Julien Grall <julien.grall@arm.com>
Acked-by: Julien Grall <julien.grall@arm.com>