i2c: designware: Add clk_{un}prepare() support

clk_{un}prepare is mandatory for platforms using common clock framework. Since
this driver is used by SPEAr platform, which supports common clock framework,
add clk_{un}prepare() support for designware i2c.

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>