Documentation: Add ARMv8 to arch_timer devicetree

Currently the documentation for the arch_timer devicetree binding only
lists "arm,armv7-timer".

Add "arm,armv8-timer" to the list of compatible strings.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>