leds: trigger: oneshot: Add initialization from Device Tree

Allow initialization of delays used in oneshot trigger from Device
Tree property.

This is especially useful for embedded systems where the trigger might
be used early, before bringing up user-space.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>