phy: Renesas R-Car Gen2 PHY driver

This PHY, though formally being a part of Renesas USBHS controller, contains the
UGCTRL2 register that controls multiplexing of the USB ports (Renesas calls them
channels) to the different USB controllers: channel 0 can be connected to either
PCI EHCI/OHCI or USBHS controllers, channel 2 can be connected to PCI EHCI/OHCI
or xHCI controllers.

This is a new driver for this USB PHY currently already supported under drivers/
usb/phy/. The reason for writing the new driver was the requirement that the
multiplexing  of USB channels to the controller be dynamic, depending on what
USB drivers  are loaded,  rather than static as provided by the old driver. The
infrastructure provided by drivers/phy/phy-core.c  seems to fit that purpose
ideally. The new driver only  supports device tree probing  for now.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>