dt-bindings: arm: qcom: Document sm7225 and fairphone,fp4 board

Add binding documentation for Fairphone 4 smartphone which is based on
Snapdragon 750G (sm7225).

Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20211007212444.328034-10-luca@z3ntu.xyz