board: librem5: Set VSYS_MIN to 3.4V

This fixes the red LED shining up (signalizing low battery)
when the battery is in fact far from being low. This happens
because bq25895 ADC can't read any voltage lower than VSYS_MIN,
so the red light used to be triggered anytime the voltage dropped
below 3.7 V.

Based on schematics and datasheets, every component connected
directly to VSYS is supposed to operate at least down to 3.3V.
1 job for vsys in 2 minutes and 30 seconds (queued for 3 seconds)
Status Name Job ID Coverage
passed build:cross:debian-buster #376121