ppc4xx: Implement is_pci_host() for 405 CPUs

This patch implements the is_pci_host() function in a similiar way
as it is used on 440 targets.

The former path with CONFIG_PCI_HOST == PCI_HOST_AUTO does not
build on 405EP targets because checking the PCI arbiter is different.
So putting the fixed code into a separate function makes the code
more readable.

Also using is_pci_host() on 405 brings 405 and 440 PCI code
a little bit closer.

In preparation for an upcoming 405EP based PMC module I made this
function weak so that it can be overwritten from board specific code.

Signed-off-by: Matthias Fuchs <matthias.fuchs@esd.eu>
Signed-off-by: Stefan Roese <sr@denx.de>