nl802154: add support for dump phy capabilities

This patch add support to nl802154 to dump all phy capabilities which is
inside the wpan_phy_supported struct. Also we introduce a new method to
dumping supported channels. The new method will offer a easier interface
and has lesser netlink traffic.

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