powerpc: Add support for non-PCI ISA bridges

The POWER9 chip supports an LPC bus that isn't hanging
off a PCI bus, so let's add support for that, mapping it
to the reserved space at ISA_IO_BASE

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>