drivers: ieee802154: cc2520: Convert to DEVICE_DT_INST_DEFINE

Move driver to use {NET_}DEVICE_DT_INST_DEFINE.  This lets us
remove the IEEE802154_CC2520_DRV_NAME Kconfig symobl.

We also update the ieee802154 build_all test to actually enable
the CC2520 driver.

Signed-off-by: Kumar Gala <galak@kernel.org>