dts: arm: Add missing adc property for kinetis k8x and kv5x socs

Adds the required #io-channel-cells property to kinetis k8x and kv5x
socs. We are now consistently defining this property for all
nxp,kinetis-adc16 compatible nodes.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>