IEEE80154: Add documentation to the IEEE80154 netlink and fakehard driver

This adds some perfunctory documentation comments to the IEEE 802.15.4
fakehard.c driver (Fake hard MAC) and the nl802154.h (outgoing netlink messages)
header.

These comments are not necessarily complete, but they do reference the
IEEE 802.15.4-2006 document where possible.

Signed-off-by: Daniel Silverstone <dsilvers@simtec.co.uk>