Add fallback compatibility string for R-Car Gen 1 and Gen2.

In the case of Renesas R-Car hardware we know that there are generations of
SoCs, e.g. Gen 1 and Gen 2. But beyond that its not clear what the
relationship between IP blocks might be. For example, I believe that
r8a7779 is older than r8a7778 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: Rob Herring <robh@kernel.org>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>