usb: host: xhci-plat: add R-Car Gen2 and Gen3 fallback compatibility strings

Add fallback compatibility strings for R-Car Gen2 and Gen3.

This is in keeping with the fallback scheme being adopted wherever
appropriate for drivers for Renesas SoCs.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>