phy: Add Tegra XUSB pad controller support

Add a new driver for the XUSB pad controller found on NVIDIA Tegra SoCs.
This hardware block used to be exposed as a pin controller, but it turns
out that this isn't a good fit. The new driver and DT binding much more
accurately describe the hardware and are more flexible in supporting new
SoC generations.

Acked-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>