Fix ide on Swarm and improve it a little

[Sebastian Andrzej Siewior <sebastian@breakpoint.cc>]

Patch #1 fixes the ide trouble on Swarm. We don't have dcache alliasing
         there but icache does not snoop dcache so we can't execute / boot
         from ide devices unless dcache is flushed.
Patch #2 is ths' old patch rebased with a few optimizations.
Patch #3 moves the dcache flush from arch code into ide subsystem. David,
         I hope I don't break any of your sparcs. The dcache flush in
         write path looks like a nop.

The whole series was tested on MIPS BCM1250 B2 aka Swarm with its default
config and CONFIG_BLK_DEV_PLATFORM=y instead PATA. PATA seems not to work
(yet).

Sebastian