arm64: dts: imx8mp-verdin: add cpu-supply

Add the cpu-supply property to all CPU nodes to enable the cpufreq
driver.

Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>