drivers: clk: st: Handle clk synchronous mode for video clocks

This patch configures the semi-synchronous mode of the video clocks
of clkgenD2.

Signed-off-by: Olivier Bideau <olivier.bideau@st.com>
Signed-off-by: Gabriel Fernandez <gabriel.fernandez@st.com>
Acked-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>