ARM: plat-nomadik: handle clocking properly

clk_prepare() was missing from the Nomadik MTU driver, also handle
errors on prepare and enable in the simplest way possible, by
bugging out - we cannot start the system without time anyway.

Acked-by: Alessandro Rubini <rubini@unipv.it>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>