dts: nxp: k6x: add temperature sensor nodes

Add device tree nodes for the internal temperature sensor in the NXP
Kinetis K6x SoC series.

A temperature sensor node is added for each ADC in the SoC to allow
the user to choose which ADC instance to use for the sensor readings.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>