sh: pci: Consolidate pci_iomap() and use the generic I/O base.

This consolidates the pci_iomap() definitions and reworks how the I/O
port base is handled. PCI channels can register their own I/O map base,
or if none is provided, the system-wide generic I/O base is used instead.

Functionally nothing changes, while this allows us to kill off lots of
I/O address special casing and lookups.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>