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

Add <linux/eventfd.h> and expose the `eventfd_*()` API. This is used by
DRM drivers for some time, but the code was commented out so far.

Note that Linux uses `struct eventfd_ctx`, but FreeBSD defines `struct
eventfd`. We define `eventfd_ctx` as a synonym to `eventfd`.

Reviewed by:	christos, markj
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50853

The amdgpu DRM driver started to include in from Linux 6.10, but either
it does not use any of it, or linuxkpi provides the necessary API from
another header.

Reviewed by:	bz
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D54498

The DRM generic code started to include it in Linux 6.10 but doesn't
seem to use anything inside.

Reviewed by:	bz
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D54490

There are no consumers right now except the upcoming <linux/eventfd.h>.
This is meant to reproduce the same includes as Linux, in case consumers
of <linux/eventfd.h> implicitly rely on namespace pollution.

Reviewed by:	bz, emaste
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50851

These are needed for mt76.

Sponsored by:	The FreeBSD Foundation
MFC after:	3 days

Long-term asm/unaligned.h is likely to migrate to this file?

Sponsored by:	The FreeBSD Foundation
MFC after:	3 days
Reviewed by:	emaste
Differential Revision: https://reviews.freebsd.org/D50003

[Why]
The i915 DRM driver relies on this chain of includes to access the
definition of `struct tasklet_struct` in `<linux/interrupt.h>`.

Reviewed by:	imp, manu
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48760

Those files are required by updated wireless drivers.

Sponsored by:	The FreeBSD Foundation
MFC after:	3 days

kstrtox.h has an implementation nowadays.  Remove the dummy file
which is no longer needed.

Sponsored by:	The FreeBSD Foundation