powerpc: reduce number of WATCHDOG_RESET calls from flush_cache

Calling WATCHDOG_RESET for each and every cache line is overkill.

In our case, the kernel image is a little over 7MB, and the almost
500000 calls of WATCHDOG_RESET() adds about one second to the
boottime.

I very highly doubt there's any real hardware where flushing 64K
from cache to memory takes more than a few milliseconds, so this
should be completely safe. Since it reduces the number of
WATCHDOG_RESET() calls by roughly a factor of 1000, the overhead from
those is practically eliminated. (Just in case the range flushed is so
small that it doesn't cross a 64K boundary, add a single
WATCHDOG_RESET() between the loops).

64K is chosen because that's also the default chunk size used by the
hashing algorithms, and when, say, a sha256 digest of a kernel image of
a few MB is being verified, that's almost guaranteed to be cache-cold,
so apart from the computations being done, the hashing is also bounded
by memory speed - so if 64K works for those cases, it should certainly
also work when memory access is the only thing being done.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>