dt-bindings: phy: qcom,m31: Document qcom,m31 USB phy

Document the M31 USB2 phy present in IPQ5332.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Co-developed-by: Sricharan Ramabadhran <quic_srichara@quicinc.com>
Signed-off-by: Sricharan Ramabadhran <quic_srichara@quicinc.com>
Signed-off-by: Varadarajan Narayanan <quic_varada@quicinc.com>
Link: https://lore.kernel.org/r/44a31cf0361df8db527684c7fb3b38e6ece950c1.1691999761.git.quic_varada@quicinc.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>