ARM: dts: am3517-som: Associate cpu to regulator supply

The am3517-som is powered by vdd_core_reg, so let's add the
'cpu' device-tree node with the 'vdd_core' regulator as the core
supply.

Signed-off-by: Derald D. Woods <woods.technical@gmail.com>
Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>