We have a few places where driver code prints reset_source_name() to
console after setting the reset source.

This working is probe order dependent, because reset_source_name()
prints the highest priority reset_source so far, which doesn't
necessarily have to be the one that was just computed.

Implement reset_source_to_string, so drivers can be migrated to use it.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>