Feature: 'default' SPI support for atmel-samd boards

[matt-land]

This extends from PR #841 (which should merge first) to add SPI support in a similar fashion as default I2C support.

import board
my_spi = board.SPI()

Boards were chosen for support for having all of the following defined in pins.c for the target board:

• MP_QSTR_SCK
• MP_QSTR_MISO
• MP_QSTR_MOSI

Boards that have support have the following in mpconfigboard.h:

• #define DEFAULT_SPI_BUS_SCK (&pin_PB11)
• #define DEFAULT_SPI_BUS_MOSI (&pin_PB10)
• #define DEFAULT_SPI_BUS_MISO (&pin_PA12)

To change default SPI for your board, change the pin targets in mpconfigboard.h

Supported boards:

• arduino_zero
• circuitplayground_express
• feather_m0_adalogger
• feather_m0_basic
• feather_m0_express
• feather_m0_rfm69
• feather_m0_rfm9x
• feather_m0_supersized
• feather_m4_express
• metro_m0_express
• metro_m4_express
• metro_m4_express_revb
• itsybitsy_m0_express
• itsybitsy_m4_express
• trinket_m0
• trinket_m0_haxpress

Not Supported:

• gemma
• pirkey
• ugame10

[tannewt]

Great! Thanks for adding SPI too!