ARM: dts: imx6sx: Use "vddarm" as the regulator name

The other i.mx6 variants use "vddarm" as the name of the internal regulator
that powers the core. Use the same convention here for consistency.

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