boards: arm: particle_boron: fix SARA-R4 gpio definitions

These definitions were left incorrect after a gpio-map change was
added to the PR introducing the boron support for the SARA-R4 modem.

Correct the gpio defintions here so that the boron build doesn't
break.

Fixes: https://github.com/zephyrproject-rtos/zephyr/issues/16289



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