IOC3: Program UART predividers.

The IOC3 driver's UART detection bits used to rely on the the firmware
setting the UART pre-divider in a way that's apropriate for the 8250
driver which doesn't currently program this register.  This happens
to work for the console but not rarely for additional ports.

While at it, also program the UART to RS-232 PIO mode; it the UART might
have been in mac-serial and/or DMA mode though that hasn't actually been
observed in practice.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>