The ARM architected timer driver doesn't compile without
GENERIC_CLOCKEVENTS selected, so ensure that we select it when building
for a platform that has the timer.

Without this patch, mach-virt fails to build without something like
mach-vexpress also selected.
Acked-by: NMarc Zyngier <marc.zyngier@arm.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>