powerpc: Move mpc83xx_add_bridge to fsl_pci.c

This allows other platforms with the same pci block like MPC5121 to use it.

Signed-off-by: John Rigby <jrigby@freescale.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>