dt-bindings: display: sii902x: Change audio mclk binding

As stated in SiL9022/24 datasheet, master clock is not required for I2S.
Make mclk property optional in DT bindings.

Fixes: 3f18021f

 ("dt-bindings: display: sii902x: Add HDMI audio bindings")

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/1563811560-29589-3-git-send-email-olivier.moysan@st.com