Only set up the interrupts that will actually be used in arm generic_timer.

The code previously set up interrupt handlers for all the interrupt
resources available, including for timers that are not in use. That could
lead to interrupt storms. For example, if boot firmware enabled the virtual
timer but the kernel is using the physical timer, it could get flooded with
interrupts on the virtual timer which it cannot shut off. By only setting
up an interrupt handler for the hardware that will actually be used, any
interrupts from other timer units will remain masked in the interrupt
controller.

Differential Revision: https://reviews.freebsd.org/D19871