ravb: add fallback compatibility strings

Add fallback compatibility strings for R-Car Gen 2 & 3 SoC Families.
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: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>