arm64: dts: qcom: msm8916: Set 'xo_board' as ref clock of the DSI PHY

Add 'xo_board' as ref clock for the DSI PHYs, it was previously
hardcoded in the PLL 'driver' for the 28nm PHY.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Andy Gross <agross@kernel.org>