ARM: dts: imx6: Use 'vddarm' as the regulator name

Instead of calling the regulator for the ARM core as 'cpu', let's rename it
as 'vddarm', so that we keep a better consistency with the other internal
regulators:

vdd1p1: 800 <--> 1375 mV at 1100 mV
vdd3p0: 2800 <--> 3150 mV at 3000 mV
vdd2p5: 2000 <--> 2750 mV at 2400 mV
vddarm: 725 <--> 1450 mV at 1150 mV
vddpu: 725 <--> 1450 mV at 1150 mV
vddsoc: 725 <--> 1450 mV at 1200 mV

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>