Add ability to read Vdd: Looking for preference in how to code the pull request

[ajs123]

For my application, I'd like to be able to read Vdd. The nrf52 allows this by selecting SAADC channel 9.

I see three options for where to put this:

1. Add readVdd() to wiring_analog_nrf52.c.
2. Add a pin for Vdd so that it can be selected in analogRead().
3. Make analogReadInternal() public in wiring_analog_nrf52.h so that channel 9 can be chosen.

I think the best way is #1 but I wanted to check before adding the code. I'll follow with a PR.

Thanks!

[lyusupov]

1. Add readVdd() to wiring_analog_nrf52.c.

Adafruit_nRF52_Arduino/cores/nRF5/wiring_analog_nRF52.c

Line 282 in 7770246

uint32_t analogReadVDD( void )

[ajs123]

I missed it! Duh!