clk: pxa clocks build system fix

Fix the building of pxa clock drivers so that the files are actually
compiled if and only if COMMON_CLK was selected by the architecture.

This prevents conflicts with mach-pxa clock legacy implementation.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Mike Turquette <mturquette@linaro.org>
diff --git a/drivers/clk/Kconfig b/drivers/clk/Kconfig
index 85131ae..1d4d3fc 100644
--- a/drivers/clk/Kconfig
+++ b/drivers/clk/Kconfig
@@ -120,6 +120,11 @@
 	  This driver supports TI Palmas devices 32KHz output KG and KG_AUDIO
 	  using common clock framework.
 
+config COMMON_CLK_PXA
+	def_bool COMMON_CLK && ARCH_PXA
+	---help---
+	  Sypport for the Marvell PXA SoC.
+
 source "drivers/clk/qcom/Kconfig"
 
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