power: reset: ltc2952: make trigger delay configurable

Make trigger delay configurable through device tree with
trigger-delay-ms property.

Trigger delay is the time to wait before starting shutdown
sequence after trigger line assertion.
Trigger delay must take into account the OFFT time configured
with the capacitor connected to OFFT pin of the LTC2952 chip.
Basically, the higher the capacitance connected to OFFT pin,
the larger trigger delay must be.

Signed-off-by: Marek Czerski <ma.czerski@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>