arm: arch_timer: move core to drivers/clocksource

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>
diff --git a/arch/arm/mach-omap2/Kconfig b/arch/arm/mach-omap2/Kconfig
index 41b581f..9d7909e 100644
--- a/arch/arm/mach-omap2/Kconfig
+++ b/arch/arm/mach-omap2/Kconfig
@@ -76,12 +76,12 @@
 
 config SOC_OMAP5
 	bool "TI OMAP5"
-	select ARM_ARCH_TIMER
 	select ARM_CPU_SUSPEND if PM
 	select ARM_GIC
 	select CPU_V7
 	select HAVE_SMP
 	select COMMON_CLK
+	select HAVE_ARM_ARCH_TIMER
 
 comment "OMAP Core Type"
 	depends on ARCH_OMAP2