max44000: Support controlling LED current output

This is exposed as an output channel with "led" as an extend_name.

Other sensors also have support for controlling an external LED. It's
not clear that simply exposing an undecorated output channel is the
correct approach.

Signed-off-by: Crestez Dan Leonard <leonard.crestez@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>