soc: fsl: cpm1: qmc: Add support for child devices

QMC child devices support is needed to avoid orphan DT nodes that use a
simple DT phandle to reference a QMC channel.

Allow to instantiate child devices and also extend the API to get the
qmc_chan using a child device.

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Link: https://lore.kernel.org/r/20231205152116.122512-7-herve.codina@bootlin.com