usb: renesas_usbhs: add support for generic PHY

This patch adds support for the generic PHY. The generic PHY will be
used in multiplatform environment.

Acked-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>