The PICO-IMX7D SoM has the usual power supply configuration using
output sw1a of PF3000 PMIC, which was defined in downstream derivative
of linux-imx (see link) in the sources for "Android Things" devkit.
It is required to support CPU frequency scaling.

Map the respective "cpu-supply" nodes of each core to sw1a of the PMIC.

Enabling them causes cpufreq-dt, and imx-thermal drivers to probe
successfully, and CPU frequency scaling to function.

Link: https://android.googlesource.com/platform/hardware/bsp/kernel/nxp/imx-v4.1/+/o-iot-preview-5/arch/arm/boot/dts/imx7d-pico.dtsi#849

Cc: Fabio Estevam <festevam@gmail.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Lech Perczak <lech.perczak@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>