ArmPkg/Drivers/TimerDxe: Improve Timer initialisation.

ArmPkg/Drivers/TimerDxe: Improve Timer initialisation.

- Registering a interrupt handler implicitly enables said interrupt. This
  is in the UEFI Spec. No need to enable the interrupts a second time.
- Make sure the Timer is completely disabled before configuring it. Only
  enable after configuration is complete.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Olivier Martin <olivier.martin@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14433 6f19259b-4bc3-4df7-8a09-765794883524