ieee802154: support 100kbps QPSK/EU in at86rf230

The standard assigns channel 0 on page 2 to be 100kbps QPSK in the
868.3MHz band. Add support to the at86rf230 driver for this channel and
page, at the moment predicated only for the RF212 chip.

Per the datasheet, configurations for page 0, channels 0 to 10 and page
2, channels 0 to 10 differ only in the BPSK_QPSK bit. Support for
channels 1 to 10 is untested.

Signed-off-by: Phoebe Buckheister <phoebe.buckheister@itwm.fraunhofer.de>
Signed-off-by: David S. Miller <davem@davemloft.net>