leds: lm3692x: Change DT calls to fwnode calls

Update the code to use the fwnode calls as opposed
to ARM DT node api's.

This allows the device to be used with either DT
configurations or ACPI definitions.

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>