The PMIC node can be a clock provider (for its 32 kHz clock) and authors
of imx8mq-librem5-devkit.dts apparently wanted this because they added
input clock and clock-output-names.

Add necessary clock-cells to the PMIC node.
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-and-tested-by: Martin Kepplinger <martink@posteo.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>