ArmPkg: allow dynamically discovered timer interrupts

To support booting on virtual machines whose interrupt routing is
discovered from the device tree, allow the interrupt numbers to
be redeclared as PcdsDynamic by the platform .dsc

Contributed-under: TianoCore Contribution Agreement 1.0
Acked-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-By: Olivier Martin <olivier.martin@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16071 6f19259b-4bc3-4df7-8a09-765794883524