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>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 0365cbb..a5d416e 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -609,6 +609,7 @@
 	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 @@
 	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