Add defines for internal DotStar pins

[caternuson]

For boards that have onboard (internal) DotStars:

• Adafruit Trinket M0
• Itsy Bitsy M0 Express
• Adafruit PyRuler
• Adafruit GEMMA M0
• Adafruit Itsybitsy m4
• others?

add #defs for the DotStar pins, ex:

// DotStar LED
#define INTERNAL_DS_DATA     3
#define INTERNAL_DS_CLK      4

to make referencing them easy in code.

EDIT: remove non-SAMD boards from copy-pasta'd list

[hathach]

Is it OK to name it as PIN_DOTSTAR_DATA, PIN_DOTSTAR_CLK, DOTSTAR_NUM to be consistent with existing neopixel naming. I could help to implement this based on schematic

https://github.com/adafruit/ArduinoCore-samd/blob/2a7aae79ee52ba95ce70ec25a551e3a60d2ce5f6/variants/circuitplay/variant.h#L94-L95

[ladyada]

yep!

[caternuson]

Thanks. Yes, let's decide name now. Easy to change to whatever at this point.

For boards that already have something, like Gemma - replace existing? or just add new defs?

[caternuson]

Done with #321