mpc83xx: MPC837xEMDS: Use hwconfig instead of pci_external_arbiter variable

Since we have simple hwconfig interface now, we don't need
pci_external_arbiter variable any longer.

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