ARM: pxa: Add CLKSRC_OF dependency

Select CLKSRC_OF for PXA architectures.
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

arch/arm/Kconfig

@@ -634,6 +634,7 @@ config ARCH_PXA
 	select AUTO_ZRELADDR
 	select CLKDEV_LOOKUP
 	select CLKSRC_MMIO
+	select CLKSRC_OF
 	select GENERIC_CLOCKEVENTS
 	select GPIO_PXA
 	select HAVE_IDE