ARM: dts: imx: Use 'pmic' as node name

Devicetree node names should be generic. Use 'pmic' as node name
to avoid devicetree check warning from pfuze100.yaml.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>