ARM: versatile: Make plat-versatile clock optional

... in preparation for common clock coming for Integrator
and Versatile Express.

Based on Linus Walleij's "ARM: integrator: convert to common
clock" patch.

Signed-off-by: Pawel Moll <pawel.moll@arm.com>
diff --git a/arch/arm/plat-versatile/Kconfig b/arch/arm/plat-versatile/Kconfig
index 81ee7cc..8d5c10a 100644
--- a/arch/arm/plat-versatile/Kconfig
+++ b/arch/arm/plat-versatile/Kconfig
@@ -1,5 +1,8 @@
 if PLAT_VERSATILE
 
+config PLAT_VERSATILE_CLOCK
+	bool
+
 config PLAT_VERSATILE_CLCD
 	bool