ARM: dts: am335x-icev2: Add ADC support

7 of the 8 ADC inputs on the am335x are connected to the Industrial I/O
header, add this here.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>