ARM: zImage: fix comments for cache_on, cache_off and cache_clean_flush

This adds missing registers to the list of corrupted registers and
removes a wrong comment about r9 on entry

While at it the formatting of the comment to cache_off is changed to
resemble the other two.

Acked-by: Eric Miao <eric.miao@canonical.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>