arm64: dts: imx8mp-evk: add regulator supply to CPU nodes

This supply is used by cpufreq to scale the voltage applied to the
ARM voltage domain of the SoC when switching between the different
operating points.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>