ieee802154: introduce sysfs file

This patch moves the sysfs handling in a own file. This is like wireless
sysfs file handling.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

diff --cc net/ieee802154/Makefile
Simple merge

diff --cc net/ieee802154/core.c
Simple merge

diff --cc net/ieee802154/sysfs.c
Simple merge

diff --cc net/ieee802154/sysfs.h
Simple merge