MIPS: Alchemy: pata_platform for DB1200

The au1xxx-ide driver isn't any faster than pata_platform since it
spends a lot of time busy waiting for DMA to finish; faster PIO/DMA
modes only work on the db1200 with a certain cpu speed, UDMA is broken,
and finally the old IDE layer is on death row, so time to switch to
the newer ATA layer.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Cc: Linux-MIPS <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/6662/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>