linuxkpi: Add <linux/mfd/core.h>

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Summary

To be exact, there was a dummy file with no content before. This commit defines struct mfd_cell and adds two function stubs.

The function stubs are not implemented but still return success. They log a message to indicate they need to be implemented.

Also, unlike Linux, <linux/mfd/core.h> includes <linux/ioport.h>. This works around the fact that we can't include <linux/ioport.h> from <linux/pci.h>, due to a conflict with the FreeBSD-native struct resource.

The amdgpu DRM driver started to use it in Linux 6.11.

This is part of the update of DRM drivers to Linux 6.11.

Sponsored by: The FreeBSD Foundation

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Fix copyright header to use SPDX comment.