8250: Serial driver changes to support future Cavium OCTEON serial patches.

In order to use Cavium OCTEON specific serial i/o drivers, we first
patch the 8250 driver to use replaceable I/O functions.  Compatible
I/O functions are added for existing iotypeS.

An added benefit of this change is that it makes it easy to factor
some of the existing special cases out to board/SOC specific support
code.

The alternative is to load up 8250.c with a bunch of OCTEON specific
iotype code and bug work-arounds.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Signed-off-by: Tomaso Paoletti <tpaoletti@caviumnetworks.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>