CC1120 has a new footprint. Add datasheet and footprint generator

CC1120 has a new footprint. Add datasheet and footprint generator

The only change from the 1120 footprint is to make the center pad a
bit smaller. The docs have a much more elaborate change, but that
involves putting vias in the middle of the solder paste area, which
seems like a bad plan unless we used a via-in-pad process.

Signed-off-by: Keith Packard <keithp@keithp.com>