dt-bindings: phy: Add binding for Qualcomm PCIe2 PHY

The Qualcomm PCIe2 PHY is a Synopsys based PCIe PHY found in a number of
Qualcomm platforms, add a binding to describe this.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>