ieee802154: Add CA8210 IEEE 802.15.4 device driver

Add driver source and config for softMAC implementation of Cascoda's CA8210
IEEE 802.15.4 transceiver device. The driver mimics a common PHY-only
implementation despite the CA8210 being a hardMAC device which exposes a SAP
interface to the fully integrated MAC.

The chip is a modem-only device with an integrated processor which runs the
802.15.4 MAC. The chip communicates via full-duplex SPI with additional pins
for NIRQ and NRESET. The chip can also output its 16MHz clock to a GPIO with a
configurable divider.

The driver can be configured to implement a debugfs node that provides access
to the SAP-based API to drive mechanisms not currently supported by the
standard kernel interface.

Signed-off-by: Harry Morris <h.morris@cascoda.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>