PCI: rcar: Add error interrupt handling

Add option to enable interrupts to report any errors from the AHB-PCI
bridge to help find any issues with the bridge when in use.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Simon Horman <horms+renesas@verge.net.au>