altos: Wire up another CC1120 GPIO to get MARC status changes

When the radio drops out of RX or TX mode due to an error, it changes
the MARC status, and sends pulse down a configured GPIO. Use this to
tell when something 'bad' happened during TX or RX so that we can
recover from losing the SPI bus in the middle of transmission or
reception.

Without this, the radio would change state and we'd never know,
leaving the radio code waiting for an interrupt that would never arrive.

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