dt-bindings: msm: dsi-phy-28nm: Document fam-b compatible

Document omitted 28nm compatible which will be used on MSM8976 SoC.
Signed-off-by: NAdam Skladowski <a39.skl@gmail.com>
Acked-by: NKrzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/516427/
Link: https://lore.kernel.org/r/20221231170532.77000-1-a39.skl@gmail.comSigned-off-by: NDmitry Baryshkov <dmitry.baryshkov@linaro.org>

Documentation/devicetree/bindings/display/msm/dsi-phy-28nm.yaml

@@ -16,6 +16,7 @@ properties:
   compatible:
     enum:
       - qcom,dsi-phy-28nm-hpm
+      - qcom,dsi-phy-28nm-hpm-fam-b
       - qcom,dsi-phy-28nm-lp
       - qcom,dsi-phy-28nm-8960