phy: rcar-gen3-usb2: Add R-Car Gen3 USB2 PHY driver

This patch adds support for R-Car generation 3 USB2 PHY driver.
This SoC has 3 EHCI/OHCI channels, and the channel 0 is shared
with the HSUSB (USB2.0 peripheral) device. And each channel has
independent registers about the PHYs.

So, the purpose of this driver is:
 1) initializes some registers of SoC specific to use the
    {ehci,ohci}-platform driver.

 2) detects id pin to select host or peripheral on the channel 0.

For now, this driver only supports 1) above.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>