Serial: UART driver changes for Cavium OCTEON.

Cavium UART implementation is not covered by existing uart_configS.
Define a new uart_config (PORT_OCTEON) which is specified by OCTEON
platform device registration code.

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