ARM: dts: imx7d: correct cpu supply name for voltage scaling

Correct CPU supply name to meet cpufreq-dt driver's
requirement for voltage scaling.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>