dt-bindings: display: msm-dsi-phy-7nm: document the QCS8300 DSI PHY

dt-bindings: display: msm-dsi-phy-7nm: document the QCS8300 DSI PHY

QCS8300 uses the same 5nm MDSS DSI PHY IP as SA8775P, sharing
an identical register layout and programming model. Introduce a
QCS8300-specific compatible with a fallback to `qcom,sa8775p-dsi-phy-5nm`
to reflect this hardware reuse.

Signed-off-by: Ayushi Makhija <quic_amakhija@quicinc.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/696783/
Link: https://lore.kernel.org/r/20260104134442.732876-2-quic_amakhija@quicinc.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>