[POWERPC] 4xx: Set ibpre for 405EX in 4xx PCIe driver

This patch sets the ibpre flag (Inbound Presence) for the 405EX
in the 4xx PCIe driver.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>