linuxkpi: Define `dev_is_platform()` and `to_platform_device()`

[dumbbell]

Summary

The former returns false and the latter returns NULL.

This is part of the update of the DRM drivers to Linux 5.14.

[manu]

What do they do exactly in Linux ?

[dumbbell]

In D38163#866986, @manu wrote:

What do they do exactly in Linux ?

I'm not sure I understand the concept, so let me copy/paste the comment in Documentation/driver-api/driver-model/platform.rst:

Platform devices are devices that typically appear as autonomous
entities in the system. This includes legacy port-based devices and
host bridges to peripheral buses, and most controllers integrated
into system-on-chip platforms.  What they usually have in common
is direct addressing from a CPU bus.  Rarely, a platform_device will
be connected through a segment of some other kind of bus; but its
registers will still be directly addressable.

In the context of DRM, it is used in the support for VESA/EFI during early stages of the boot. The code is not called on FreeBSD.

[manu]

Ok, good to me then, but please add in the commit message that those are no-op just to satisfy drm compiling.