ieee802154: cc2520: Externalize how GPIOs are configured

Currently it's only possible to configure those GPIOs through
boards.<h/c> files and thus it's not relevant for board that do not
embed cc2520 but might get one wired to it, unlike
quark_se_c1000_devboard which directly embeds one cc2520.

Change-Id: I819bc1d2de707ea12eb70dc60a40b28f92666e51
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>