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/drivers/clocksource/Kconfig b/drivers/clocksource/Kconfig
index 4117386..d4221e0 100644
--- a/drivers/clocksource/Kconfig
+++ b/drivers/clocksource/Kconfig
@@ -307,8 +307,7 @@
 	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.