ieee802154: cc2520: Move to DT only config support

Now that the in tree user of cc2520 uses device tree to configure SPI
and GPIO params, we can remove and convert the driver to utilize DT
only.  This means removing the Kconfig options that come from DT and
rename CONFIG_ to DT_ for those options.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>