arm/arm64: timer: Extract irqs at setup time

The timer can be useful for other tests besides the timer test.
Extract the DT parsing of the irqs out of the timer test into
setup and provide them along with some defines in a new timer.h
file.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>