drivers: sensors: add fcx-mldx5 o2 sensor

Add driver for Angst+Pfister O2 sensors FCX-MLD25 & FCX-MLD95 and maybe
more. Tested with FCX-MLD25.

Supports get O2 value, get status, and power management.

Note that in suspended power mode heating output is at 20 %, thus
probably not suited for a battery powered device.

Signed-off-by: Jeppe Odgaard <jeppe.odgaard@prevas.dk>