mpc8572 additional end-point mode

mpc8572 supports all pcie controllers as end-points with cfg_host_agent=0.
Include host_agent == 0 decode for end-point determination.

This is not needed for the ds reference board since pcie3 will be a host
in order to connect to the uli chip.  Include it here as a reference for
other mpc8572 boards.

Signed-off-by: Ed Swarthout <Ed.Swarthout@freescale.com>