dt-bindings: phy-qcom-ipq4019-usb: add binding document

This patch adds the binding documentation for the HS/SS USB PHY found
inside Qualcom Dakota SoCs.

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Robert Marko <robert.marko@sartura.hr>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Luka Perkov <luka.perkov@sartura.hr>
Link: https://lore.kernel.org/r/20200503201823.531757-2-robert.marko@sartura.hr
Signed-off-by: Vinod Koul <vkoul@kernel.org>