mpc83xx: fix PCI scan hang on the standalone MPC837xE-MDS boards

mpc83xx: fix PCI scan hang on the standalone MPC837xE-MDS boards

The MPC837xE-MDS board's CPLD can auto-detect if the board is on the PIB,
standalone or acting as a PCI agent. User's Guide says:

- When the CPLD recognizes its location on the PIB it automatically
  configures RCW to the PCI Host.
- If the CPLD fails to recognize its location then it is automatically
  configured as an Agent and the PCI is configured to an external arbiter.

This sounds good. Though in the standalone setup the CPLD sets PCI_HOST
flag (it's ok, we can't act as PCI agents since we receive CLKIN, not
PCICLK), but the CPLD doesn't set the ARBITER_ENABLE flag, and without
any arbiter bad things will happen (here the board hangs during any config
space reads).

In this situation we must disable the PCI. And in case of anybody really
want to use an external arbiter, we provide "pci_external_aribter"
environment variable.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>