linuxkpi: drm-kmod debugfs support

This diff extends LinuxKPI to support simple attribute files in debugfs.
These simple attributes are an essential component for compiling drm-kmod
with CONFIG_DEBUG_FS enabled.
This will allow for easier graphics driver debugging using
Intel's igt-gpu-tools.

Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D35883
Sponsored by: Google, Inc. (GSoC 2022)