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

Document omitted 28nm compatible which will be used on MSM8976 SoC.

Signed-off-by: Adam Skladowski <a39.skl@gmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/516427/
Link: https://lore.kernel.org/r/20221231170532.77000-1-a39.skl@gmail.com


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>