KVM: arm/arm64: timer: Rework data structures for multiple timers

Prepare for having 4 timer data structures (2 for now).

Move loaded to the cpu data structure and not the individual timer
structure, in preparation for assigning the EL1 phys timer as well.

Signed-off-by: Christoffer Dall <christoffer.dall@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>