Currently the arch_timer register accessors are thrown together with
the main driver, preventing us from porting the driver to other
architectures.

This patch moves the register accessors into a header file, as with
the arm64 version. Constants required by the accessors are also moved.
Signed-off-by: NMark Rutland <mark.rutland@arm.com>
Acked-by: NCatalin Marinas <catalin.marinas@arm.com>
Acked-by: NMarc Zyngier <marc.zyngier@arm.com>
Acked-by: NSantosh Shilimkar <santosh.shilimkar@ti.com>