Add Sercom defs for SPI and Wire in variant

[aethaniel]

This PR allows people working on their own SAMD21 based variant to use different Sercom for either SPI or Wire.

This PR solves the two concerns pointed in #3

Contains 5 atomic commits.

[aethaniel]

@smkkHw, @cpino, this PR implies a quick update of ASME variant.
I can do it if you don't have time.

[smkkHw]

YEs sure, Thank.

[aethaniel]

@smkkHw, PR done here: ameltech/sme-arduino-core#4

[cmaglie]

@aethaniel has added two commits with defaults for SPI and Wire, so updating the Zero core will not break derivatives.

[cmaglie]

@ArduinoBot build this please

[ArduinoBot]

✅ Build completed.

⬇️ Build URL: http://downloads.arduino.cc/PR/samd/package_samd-b23_index.json

ℹ️ To test this build:

1. Open the Preferences of the Arduino IDE.
2. Add the Build URL above in the Additional Boards Manager URLs field, and click OK.
3. Open the Boards Manager (menu Tools->Board->Board Manager...)
4. Install Arduino SAMD core - Pull Request Add Sercom defs for SPI and Wire in variant #24
5. Select one of the boards under SAMD Pull Request Add Sercom defs for SPI and Wire in variant #24 in Tools->Board menu
6. Compile/Upload as usual

[smkkHw]

Tested with SmartEverything Fox.
The two examples work as expected:

1. SigFox Example test the internal ASME Usart
2. NFC Example uses the I2C Bus

[pixelpixi]

I tested mapping Wire to SERCOM0 in my board's variant. Works great, thanks!