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

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.

Reviewed by: bz, emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D55736