phy: rcar-gen2: add fallback binding

In the case of Renesas R-Car hardware we know that there are generations of
SoCs, e.g. Gen 2 and Gen 3. But beyond that its not clear what the
relationship between IP blocks might be. For example, I believe that
r8a7790 is older than r8a7791 but that doesn't imply that the latter is a
descendant of the former or vice versa.

We can, however, by examining the documentation and behaviour of the
hardware at run-time observe that the current driver implementation appears
to be compatible with the IP blocks on SoCs within a given generation.

For the above reasons and convenience when enabling new SoCs a
per-generation fallback compatibility string scheme being adopted 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: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>

drivers/phy/phy-rcar-gen2.c

@@ -195,6 +195,7 @@ static const struct of_device_id rcar_gen2_phy_match_table[] = {
 	{ .compatible = "renesas,usb-phy-r8a7790" },
 	{ .compatible = "renesas,usb-phy-r8a7791" },
 	{ .compatible = "renesas,usb-phy-r8a7794" },
+	{ .compatible = "renesas,rcar-gen2-usb-phy" },
 	{ }
 };
 MODULE_DEVICE_TABLE(of, rcar_gen2_phy_match_table);