power: reset: ltc2952: make trigger input optional

Currently the ltc2952 supports only one button sequence to initiate
powerdown. This is not always desirable, as even prolonged button
presses can happen in use.

Allow ltc2952 users to pick their own power down sequence, by making the
trigger input optional. Since this still means that the ltc2952 may
power down the platform if the power button is pressed for about 5
seconds, we still need to make sure to start the watchdog toggle to
prolong the system power for as long as we need it.

This will still allow the system to control power using the kill signal.

Signed-off-by: Frans Klaver <frans.klaver@xsens.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>