ARM: dts: am57xx-idk-common: Hook smps12 regulator as cpu vdd-supply

am574x-idk has no cpu vdd-supply at the moment. Hence hook smps12
regulator as cpu vdd-supply in am57xx-idk-common as the same regulator
feeds on to cpu on am571/2/4-idks. So remove all the individual
instances and place that in common place.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>