arm64: dts: imx8mn-evk: Add cpu-supply to enable cpufreq

PMIC driver is ready on i.MX8MN EVK board, assign cpu-supply for
each A53 and restore the operating points table to enable cpufreq.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>