boards: arm: Make Adafruit boards use their DT-defined code partion.

Adafruit Feather M0 and Adafruit Trinket M0 boards aren't set up
to use their defined flash partition table to generate the 0x2000
flash offset for code. This should be enabled by default as the
boards ship with a write-protected bootloader and code should be
flashed where it expected by default.

Signed-off-by: Kuba Sanak <contact@kuba.fyi>