arch: arm: cortex_a_r: add support for arm arch timer

This include make possible to use the arm_arch_timer on
platform such as Cortex-A9 or Cortex-R7 which has support for
ARM Global Timer.

The global timer is a 64 bit incrementing counter, memory-mapped
in the private memory region.

Signed-off-by: Julien Massot <julien.massot@iot.bzh>