MIPS: Alchemy: UARTs are of type 16550A

UART autodetection breaks on the Au1300 but the IP blocks are identical,
at least according to the datasheets.  Help the 8250 driver by passing
on uart type information via platform data.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>