MIPS: Alchemy: Enable PATA_PLATFORM support

We have limited demand for platform PATA support across MIPS platforms:

$ find arch/mips -type f | sort | xargs grep -l pata_platform_info
arch/mips/alchemy/devboards/db1200.c
arch/mips/alchemy/devboards/db1300.c
arch/mips/sibyte/swarm/platform.c
$

certainly not high enough to justify enabling support for PATA_PLATFORM
port-wide.  SiByte platforms are handled selectively already, so just
make a similar arrangement for Alchemy DB1XXX platforms.

Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>