Add device_battery_num_cells

Currently in firmware we hard-code NUM_CELLS (usually to 1), and our code for calculating battery charge uses this variable in ways like "voltage = v / NUM_CELLS;"

If the battery voltage is one cell around our 4V normal, that works fine. 

However, as reported in meshtastic/firmware#5360 it's not uncommon for solar installs that have multiple batteries set up in a way that looks like 1 cell at 12V to our current code. So, battery charge % is always 100%.

Allowing num_cells as a configuration option will allow those users to specify how many batteries they have and receive accurate readings.

meshtastic/config.proto

@@ -430,6 +430,11 @@ message Config {
      */
     uint32 device_battery_ina_address = 9;
 
+    /*
+     * Number of battery cells in use. Battery charge is calculated using (v / num_cells).
+     */
+    uint32 device_battery_num_cells = 10;
+
     /*
      * If non-zero, we want powermon log outputs.  With the particular (bitfield) sources enabled.
      * Note: we picked an ID of 32 so that lower more efficient IDs can be used for more frequently used options.