sun6i: cpu_reset: Do not return from cpu_reset()

Currently on sun6i after a "reset" the prompt returns and the user can
even type stuff until the watchdog triggers and does the actual reset.

This is somewhat unexpected behavior for the "reset" command, this
commit adds an endless loop to wait for the watchdog to trigger so that
we do not return to the prompt.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>