ARM: ux500: Fix u9540 booting issues

The u9540 stopped booting after the v3.7 merge window due to
a lack of common clk support and early PRCMU initialisation.
In this patch we rectify these issues, placing the u9540
development board back into a successfully booting state.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>