PCI: rcar: Add gen3 fallback compatibility string for pcie-rcar

Add fallback compatibility string for the  R-Car Gen 3 family.  This is in
keeping with the both the existing fallback compatibility string for the
R-Car Gen 2 family and the fallback scheme being adopted wherever
appropriate for drivers for Renesas SoCs.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>