powerpc: Initialise pci_io_base as early as possible

Commit d6a9996e ("powerpc/mm: vmalloc abstraction in preparation for
radix") turned kernel memory and IO addresses from #defined constants to
variables initialised at runtime.

On PA6T (pasemi) systems the setup_arch() machine call initialises the
onboard PCI-e root-ports, and uses pci_io_base to do this, which is now
before its value has been set, resulting in a panic early in boot before
console IO is initialised.

Move the pci_io_base initialisation to the same place as vmalloc ranges
are set (hash__early_init_mmu()/radix__early_init_mmu()) - this is the
earliest possible place we can initialise it.

Fixes: d6a9996e

 ("powerpc/mm: vmalloc abstraction in preparation for radix")
Reported-by: Christian Zigotzky <chzigotzky@xenosoft.de>
Signed-off-by: Darren Stevens <darren@stevens-zone.net>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
[mpe: Add #ifdef CONFIG_PCI, massage change log slightly]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>