net/stmmac: Use clk_prepare_enable and clk_disable_unprepare

This patch fixes an issue introduced by commit ID 6a81c26f
[net/stmmac: remove conditional compilation of clk code], which
switched from the internal stmmac_clk_{en}{dis}able calls to
clk_{en}{dis}able. By this, calling clk_prepare and clk_unprepare
was removed.

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. Otherwise
the clocks are not correctly en-/disabled and ethernet support doesn't
work.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>