powerpc: Enable /dev/port when isa_io_special is set

isa_io_special is set when the platform provides a "special"
implementation of inX/outX via some FW interface for example.

Such a platform doesn't need an ISA bridge on PCI, and so /dev/port
should be made available even if one isn't present.

This makes the LPC bus IOs accessible via /dev/port on PowerNV Power8

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>