usb: host: xhci: rcar: add a new firmware version for r8a7796

This patch adds a new firmware version "V3" for r8a7796. This patch
also adds explanation of each version's information.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>