clk: versatile: make config option boolean

There is no choice to have the RealView clocks as module for sure,
so turn this config option into a boolean.

Reported-by: Pawel Moll <pawel.moll@arm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>