[p2][photon2] Add VBAT_MEAS pin and charging indication pin for Photon2

[YutingYou]

Problem

This PR is to add VBAT_MEAS and charging pin to P2 pinmap.

• VBAT_MEAS is an ADC-dedicated pin, it can measure the input voltage with 5V tolerance,
• Charging pin is only used on Lightcycle

Solution

Add VBAT_MEAS as the A6 pin, note that the reference voltage is 5V instead of 3.3V

Steps to Test

1. Re-compile and download bootloader/system-part1/user-app
2. Measure different voltage with A6 pin

Example App

#include "Particle.h"

SYSTEM_MODE(MANUAL);

Serial1LogHandler log(115200, LOG_LEVEL_ALL);

void setup() {
}

void loop() {
    int value = analogRead(A6);
    Log.info("A6 value: %d, volt: %f", value, value / 4096.0 * 5.0);

    int A0Value = analogRead(A0);
    Log.info("A0 Value: %d, volt: %f", A0Value, A0Value / 4096.0 * 3.3);
    delay(1000);
}

References

Links to the Community, Docs, Other Issues, etc..

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Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)