ieee802154: cc2520: quark_se_c1000_devboard: Move all GPIOs to DTS

Added support to the ti,cc2520 binding for optional GPIO signals and
moved the quark_se_c1000_devboard to define those signals in the DTS.
This lets us remove board.h and some #defines & fake Kconfig symbols
from the quark_se_c1000_devboard board.

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