drivers: ieee802154: rf2xx: Add support to Sub-Giga

Add support to at86rf212[b] sub-giga devices. This work enables use of
pages 0, 2 and 5 in accordance with IEEE-802.15.4/2003/2006/2011. The
proprietary speeds can be object of future work.

Note: It is recommended that user define a power table for better
performance, low emissions and to save power. A reference power table
can be found in the datasheet and should be used for tests only and
not on a final product.

Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>