ppc4xx: Enable PCIe support without PCI support on PPC440/460

By not defining CONFIG_SYS_PCI_MASTER_INIT and CONFIG_SYS_PCI_TARGET_INIT,
PCI support (host and adapter) will not be enabled. But it's still
possible to use the U-Boot PCI infrastructure for the PCIe ports.

This configuration option is needed for a new 460GT board, which uses
PCIe but has PCI disabled.

Signed-off-by: Stefan Roese <sr@denx.de>