USB: ehci: ohci: Add clk_{un}prepare() support

clk_{un}prepare is mandatory for platforms using common clock framework. Since
these drivers are used by SPEAr platform, which supports common clock framework,
add clk_{un}prepare() support for them.

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>