SPEAr: Call clk_prepare() before calling clk_enable

With common clock framework, it is must to call clk_{un}prepare() before/after
clk_{dis}enable. This patch fixes this for SPEAr timer.

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Reviewed-by: Mike Turquette <mturquette@linaro.org>