ArmPkg: add reenable hook to ArmGenericTimerCounterLib

In preparation of selectively reinstating the timer enable quirk for Xen
that we removed in commit 411a373ed6 ("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>

ArmPkg/Drivers/TimerDxe/TimerDxe.c

@@ -337,6 +337,7 @@ TimerInterruptHandler (
 
     // Set next compare value
     ArmGenericTimerSetCompareVal (CompareValue);
+    ArmGenericTimerReenableTimer ();
     ArmInstructionSynchronizationBarrier ();
   }
 

ArmPkg/Include/Library/ArmGenericTimerCounterLib.h

@@ -22,6 +22,12 @@ ArmGenericTimerEnableTimer (
   VOID
   );
 
+VOID
+EFIAPI
+ArmGenericTimerReenableTimer (
+  VOID
+  );
+
 VOID
 EFIAPI
 ArmGenericTimerDisableTimer (

ArmPkg/Library/ArmGenericTimerPhyCounterLib/ArmGenericTimerPhyCounterLib.c

@@ -29,6 +29,14 @@ ArmGenericTimerEnableTimer (
   ArmWriteCntpCtl (TimerCtrlReg);
 }
 
+VOID
+EFIAPI
+ArmGenericTimerReenableTimer (
+  VOID
+  )
+{
+}
+
 VOID
 EFIAPI
 ArmGenericTimerDisableTimer (

ArmPkg/Library/ArmGenericTimerVirtCounterLib/ArmGenericTimerVirtCounterLib.c

@@ -29,6 +29,14 @@ ArmGenericTimerEnableTimer (
   ArmWriteCntvCtl (TimerCtrlReg);
 }
 
+VOID
+EFIAPI
+ArmGenericTimerReenableTimer (
+  VOID
+  )
+{
+}
+
 VOID
 EFIAPI
 ArmGenericTimerDisableTimer (