[POWERPC] kill isa_{io,mem}_base definitions for !PCI

When CONFIG_PCI is disabled, the definitions for isa_io_base,
isa_mem_base and pci_dram_offset are entirely unused, but they
can result in link failure because they are defined in multiple
places.

The easiest fix is to just remove all these definitions.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>