cc2520: Make SPI related info configurable via Kconfig

SPI, to control the CC2520 chip, is the only generic feature and thus
the only one configurable through Kconfig. GPIO on the other end depends
a lot on the SoC/Board. Adding a slave select option as well.

Change-Id: I63068fab476ed8d5b26103e4ad20e5be253c9932
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>