boards: arm: particle_boron: enable SARA-R4 modem

The Particle.io Boron is an nRF52840-based board with a connected
u-blox SARA-R4 modem.  The main board was previously upstreamed
without modem support.

Now that we have a driver to support the SARA-R4 modem, let's enable
it for the Boron board.

Signed-off-by: Michael Scott <mike@foundries.io>