dt-bindings: phy: qcom,qmp: Describe phy@ subnode properly

Currently the qcom,qmp-phy dt-binding doesn't describe
the 'reg' and '#phy-cells' properties for the phy@ subnode.

Fix the same.

Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: Rob Herring <robh@kernel.org>
Signed-off-by: Bhupesh Sharma <bhupesh.sharma@linaro.org>