ARM: timer-sp: convert to use CLKSRC_OF init

This adds CLKSRC_OF based init for sp804 timer. The clock initialization is
refactored to support retrieving the clock(s) from the DT.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 5bfd584..10e3053 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -1173,6 +1173,7 @@
 config ARM_TIMER_SP804
 	bool
 	select CLKSRC_MMIO
+	select CLKSRC_OF if OF
 	select HAVE_SCHED_CLOCK
 
 source arch/arm/mm/Kconfig