setMinimumSystemVoltage: allow values below 3.0V on non LiPo

[g6094199]

I see the min voltage for the sysvol setting is 3.0v. i can successfully run MKR WAN 1310 as low as 2.6-2.7V even with one sensor attached. i use 18650 which have 2.5V as min voltage. it is ok to uncharge up to 2.7v

so it would be nice to have the chance to set 2.7v as min voltage.

bool PMICClass::setMinimumSystemVoltage(float voltage) {

    int DATA = readRegister(POWERON_CONFIG_REGISTER);

    if (DATA == -1) {
        return 0;
    }

    byte mask = DATA & 0xF0;
    if (voltage > 3.7f) {
        voltage = 3.7f;
    } else if (voltage < 3.0f) {
        voltage = 3.0f;
    }

    return writeRegister(POWERON_CONFIG_REGISTER, (mask | ((round((voltage - 3.0) * 10) * 2) + 1)));
}