usb: phy: registering Tegra USB PHY as platform driver

Registered Tegra USB PHY as a separate platform driver.

To synchronize host controller and PHY initialization, used deferred
probe mechanism. As PHY should be initialized before EHCI starts running,
deferred probe of Tegra EHCI driver till PHY probe gets completed.

Got rid of instance number based handling in host driver.

Made use of DT params to get the PHY Pad registers.

Signed-off-by: Venu Byravarasu <vbyravarasu@nvidia.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>