PCI: rcar: Add R8A7794 support

Add Renesas R8A7794 SoC support to the Renesas R-Car gen2 PCI driver.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Bjorn Helgaas <helgaas@kernel.org>
Acked-by: Simon Horman <horms+renesas@verge.net.au>