usb: host: xhci-plat: add firmware_name for R-Car Gen3

To clean up the xhci-rcar.c code later, this patch adds firmware_name
"V3" for R-Car Gen3.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>