target-ppc: Handle memory-forced I/O controller access

On at least the PowerPC 601, a direct-store (T=1) with bus unit ID 0x07F
is special-cased as memory-forced I/O controller access. It is supposed
to be checked immediately if T=1, bypassing all protection mechanisms
and acting cache-inhibited and global.

Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>

Simplified by avoiding reindentation. Added explanatory comments.

Cc: Alexander Graf <agraf@suse.de>
Signed-off-by: Andreas Färber <andreas.faerber@web.de>
Signed-off-by: Alexander Graf <agraf@suse.de>
