clocksource/drivers/pxa_timer: Move the Kconfig rule

Instead of having the clocksource's Kconfig depending on the arch, let the
arch to select the timer it needs.

The CLKSRC_OF dependency is removed because already selected by the
ARCH_PXA, and it is added for SA1100.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

arch/arm/Kconfig

@@ -609,6 +609,7 @@ config ARCH_PXA
 	select AUTO_ZRELADDR
 	select COMMON_CLK
 	select CLKDEV_LOOKUP
+	select CLKSRC_PXA
 	select CLKSRC_MMIO
 	select CLKSRC_OF
 	select GENERIC_CLOCKEVENTS
@@ -648,6 +649,8 @@ config ARCH_SA1100
 	select ARCH_SPARSEMEM_ENABLE
 	select CLKDEV_LOOKUP
 	select CLKSRC_MMIO
+	select CLKSRC_PXA
+	select CLKSRC_OF if OF
 	select CPU_FREQ
 	select CPU_SA1100
 	select GENERIC_CLOCKEVENTS

drivers/clocksource/Kconfig

@@ -307,8 +307,7 @@ config CLKSRC_TANGO_XTAL
 	select CLKSRC_OF
 
 config CLKSRC_PXA
-	def_bool y if ARCH_PXA || ARCH_SA1100
-	select CLKSRC_OF if OF
+	bool
 	help
 	  This enables OST0 support available on PXA and SA-11x0
 	  platforms.