Fixes:

In file included from arch/arm/include/asm/arch_timer.h:10:0,
                 from arch/arm/mach-shmobile/timer.c:23:
include/clocksource/arm_arch_timer.h:56:28: warning: 'arch_timer_get_timecounter' defined but not used [-Wunused-function]

Signed-off-by: Olof Johansson <olof@lixom.net>

The core functionality of the arch_timer driver is not directly tied to
anything under arch/arm, and can be split out.

This patch factors out the core of the arch_timer driver, so it can be
shared with other architectures. A couple of functions are added so
that architecture-specific code can interact with the driver without
needing to touch its internals.

The ARM_ARCH_TIMER config variable is moved out to
drivers/clocksource/Kconfig, existing uses in arch/arm are replaced with
HAVE_ARM_ARCH_TIMER, which selects it.

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>