LinuxKPI: add (skeleton) functions to make drivers compile

Add more functions to netdevice.h (netif_napi_add_tx() being the only
one implemented) and add platform_device.h and netlink.h in order to
make driver code compile.
The skeleton functions are used only in very limited scope and not at
all in our usage so far but add (invasive) #ifdef if removed.
Add pr_debug() calls to each of them in order to log a TODO (if DEBUG
compiled in) and someone should hit them in the future.

Commented on by: hselasky (earlier version)
Differential Revision: https://reviews.freebsd.org/D37599

(cherry picked from commit 75388b9ca5e566fb7cbd62a37cea1f66d8203fc6)