overlays: Add maxtherm overlay for MAX6675/31855

Add an overlay - maxtherm - to support the MAX6675 and MAX31855 family
of thermocouples.

Developed from an original set of overlays by Dougie Lawson.

See: https://github.com/raspberrypi/linux/pull/3763



Signed-off-by: Phil Elwell <phil@raspberrypi.com>