i.MX SHMEM-NET implementations

A network device to communicate with another domain.
Communication is done through shared memory
 and synchronized by Mailbox Units of imx.
Use Mailbox Units side A to communicate with side B.

Required properties:
- compatible : "fsl,imx-shmem-net".
- rxfirst: The flag that indicates the position of the RX buffer,
  one of the two partitions must set it.
- mub-partition: The id number of the remote processors,
  used on i.mx8qm for partition reset. The default
  value is 3 in driver without this property.
- mbox-names: the mailbox channel names.
- mboxes: the mailboxes list containing:
  MU name, channel type (0 for TX, 1 for RX) and channel id.
- memory-region: the coherent memory shared across domains.

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Example:

imx_shmem_net: imx_shmem_net {
	compatible = "fsl,imx-shmem-net";
	mub-partition = <1>;
	mbox-names = "tx", "rx";
	mboxes = <&lsio_mu8b 0 1
			  &lsio_mu8b 1 1>;
	status = "disabled";
};

&imx_shmem_net{
	memory-region = <&shmem_dma_reserved>;
	status = "okay";
};