ppc flush_cache: add watch-dog triggering into the loops.

Some boards (e.g. lwmon5) need rather a frequent watch-dog
kicking. Since the time it takes for the flush_cache() function
to complete its job depends on the size of data being flushed, one
may encounter watch-dog resets on such boards when, for example,
download big files over ethernet.

Signed-off-by: Yuri Tikhonov <yur@emcraft.com>