[PPC440SPe] PCIe environment settings for Katmai and Yucca

- 'pciconfighost' is set by default in order to be able to scan bridges
behind the primary host/PCIe

- 'pciscandelay' env variable is recognized to allow for user-controlled
delay before the PCIe bus enumeration; some peripheral devices require a
significant delay before they can be scanned (e.g. LSI8408E); without the
delay they are not detected

Signed-off-by: Grzegorz Bernacki <gjb@semihalf.com>