cc2520: Make the driver more in line with Zephyr

CC2520 drivers as a singleton:
Current driver can only handle a single instance. This is due to the
gpio callback which cannot provide the concerned device (not the gpio
device, the cc2520 one). Setting a singleton instance for now, as the
network stack cannot handle more net devices anyway. This will be fixed
in the future.
In the mean time: improving a bit the usage of spi and gpio by accessing
the cc2520 instance directly.

Also: simplifying SPI usage, useless internal locking removed, better
debugging routines, better busy wait macro and use Zephyr's BIT() macro
instead of internal one.

Change-Id: I92b849135a92f77ee6a4374c9f662dcad8347814
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>