usb: renesas_usbhs: add 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: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>