dt-bindings: usb: renesas_usbhs: add bindings for r8a77990

This patch adds bindings for r8a77990 (R-Car E3).

Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>