drm/bridge: sii902x: make audio mclk optional

The master clock on i2s bus is not mandatory,
as sii902X internal PLL can be used instead.
Make use of mclk optional.

Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
Reviewed-by: Jyri Sarha <jsarha@ti.com>
Acked-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1563811560-29589-4-git-send-email-olivier.moysan@st.com