ARM: fix missing CLKSRC_OF on multi-platform

In commit ddb902cc (ARM: centralize common multi-platform kconfig
options), CLKSRC_OF was removed from some platforms, but not added to
ARCH_MULTIPLATFORM. Fix this.
Reported-by: Lauri Hintsala <lauri.hintsala@bluegiga.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

arch/arm/Kconfig

@@ -311,6 +311,7 @@ config ARCH_MULTIPLATFORM
 	select ARM_HAS_SG_CHAIN
 	select ARM_PATCH_PHYS_VIRT
 	select AUTO_ZRELADDR
+	select CLKSRC_OF
 	select COMMON_CLK
 	select GENERIC_CLOCKEVENTS
 	select MULTI_IRQ_HANDLER