Pinctrl/spear: plgpio: don't call prepare/unprepare

SPEAr SoC's don't do anything in clk_prepare() of plgpio driver,
so there is no need to call this routine multiple times. Just
call it once at probe.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>