samd/boards: Add ADAFRUIT_FEATHER_M4_EXPRESS and _ITSYBITSY_M0_EXPRESS.

These two boards are used for testing, so it is favorable to have them
added early.

The full test set is:
- ADAFRUIT_FEATHER_M4_EXPRESS: SAMD51 with 32kHz crystal.
- ADAFRUIT_ITSYBITSY_M0_EXPRESS: SAMD21 without crystal.
- ADAFRUIT_ITSYBITSY_M4_EXPRESS: SAMD51 without crystal.
- SEEED_XIAO: SAM21 with 32kHz crystal.

ports/samd/boards/ADAFRUIT_ITSYBITSY_M0_EXPRESS/pins.csv

@@ -0,0 +1,40 @@
+# Pin rows contain Pin number and pin name.
+# Pin rows start with PIN_
+# LED rows start with LED_
+# If the pin name is omitted, the pin number is added as name.
+# Rows for empty entries have to start with '-'
+# Empty lines and lines not starting with PIN_ or LED_ are ignored
+
+PIN_PA11,D0
+PIN_PA10,D1
+PIN_PA14,D2
+PIN_PB09,D3
+PIN_PA08,D4
+PIN_PA15,D5
+-
+PIN_PA21,D7
+-
+PIN_PA07,D9
+PIN_PA18,D10
+PIN_PA16,D11
+PIN_PA19,D12
+PIN_PA17,D13
+PIN_PA02,A0
+PIN_PB08,A1
+PIN_PB09,A2
+PIN_PA04,A3
+PIN_PA05,A4
+PIN_PB02,A5
+PIN_PA22,SDA
+PIN_PA23,SCL
+PIN_PB10,MO
+PIN_PA12,MI
+PIN_PB11,SCK
+PIN_PA00,DOTSTAR_CLK
+PIN_PA01,DOTSTAR_DATA
+PIN_PB22,FLASH_MOSI
+PIN_PB03,FLASH_MISO
+PIN_PB23,FLASH_SCK
+PIN_PA27,FLASH_CS
+
+LED_PA17,LED