at86rf230: add help and 212 to Kconfig menu entry

Since commit 8fad346f
(ieee802154: add basic support for RF212 to at86rf230 driver)

we support at86rf212 as well.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>