Input: spear-keyboard - add clk_{un}prepare() support

clk_{un}prepare is mandatory for platforms using common clock framework.
Because for SPEAr we don't do anything in clk_{un}prepare() calls, just
call them once in probe/remove.

Signed-off-by: Vipul Kumar Samar <vipulkumar.samar@st.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>