split mpc8xx hooks from cmd_ide.c

move most of mpc8xx hooks from cmd_ide.c into ide_preinit() and newly created
ide_init_postreset() (invoked after calling ide_reset after ide_preinit),
some cleanup to make checkpatch happy, enable IDE init hooks in configs of
affected boards.
confusingly, these hooks are used by more than just mpc8xx-based boards, and
therefore are placed in arch/ppc/lib/

note: checkpatch still emits warnings about using volatile

Signed-off-by: Pavel Herrmann <morpheus.ibis@gmail.com>