ARM: shmobile: Enable PCI domains for R-Car Gen2 devices

The PCI core will soon automatically handle the PCI domain number,
allowing the internal PCI and external PCIe bridges work at the same time.
In order for that to work, we need to enable PCI_DOMAINS.

Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>