MIPS: enable serial UART support on PNX833X devices.

Enabled serial UART driver for PNX833X devices.

Signed-off-by: Ihar Hrachyshka <ihar.hrachyshka@gmail.com>
Signed-off-by: Alan Cox <number6@the-village.bc.nu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>