ARM: KVM: arch_timer: use symbolic constants

In clocksource/arm_arch_timer.h we define useful symbolic constants.
Let's use them to make the KVM arch_timer code clearer.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Cc: Christoffer Dall <cdall@cs.columbia.edu>
Signed-off-by: Christoffer Dall <cdall@cs.columbia.edu>