PCI: rcar: Register each instance independently

Convert the code to allow per-device probe() like other device drivers.
This also delays driver registration due to change from subsys_initcall()
to regular module_platform_driver().

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>