ARM: dts: qcom: msm8974: Add xo_board reference clock to DSI0 PHY

According to YAML validation, and for a future patchset putting this
xo_board reference clock to use as VCO reference parent, add the missing
clock to dsi_phy0.

Fixes: 5a9fc531 ("ARM: dts: msm8974: add display support")
Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20210830175739.143401-1-marijn.suijten@somainline.org