powerpc: 83xx: pci: Remove need for get_immrbase from mpc83xx_add_bridge.

Modify mpc83xx_add_bridge to get config space register base address from
the device tree instead of immr + hardcoded offset.

83xx pci nodes have this change:
    register properties now contain two address length tuples:
	First is the pci bridge register base, this has always been there.
	Second is the config base, this is new.

This is documented in dts-bindings/fsl/83xx-512x-pci.txt

The changes accomplish these things:
    mpc83xx_add_bridge no longer needs to call get_immrbase
    it uses hard coded addresses if the second register value is missing

Signed-off-by: John Rigby <jrigby@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>