ARM: architected timers: add DT support

Add runtime DT support and documentation for the Cortex A7/A15
architected timers.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

Conflicts:
        [Resolve conflicts for adding support for the feature
         ARCH_HAS_READ_CURRENT_TIMER]
	arch/arm/kernel/arch_timer.c

Change-Id: I4b1d1dc2a8c69466497423475f7a3dd4d2c380c1
Signed-off-by: Sathish Ambley <sambley@codeaurora.org>