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>