This commit adds the asynchronous UART API testing support on the SAM
R21 Xplained Pro board.
The SERCOM3 module is used as the secondary loop-back UART, which is
required to run this test.
Note that no external UART loop-back connection is necessary to run
this test, because the SERCOM3 UART TX and RX pads are configured to be
internally connected; it is, however, still necessary to configure the
pinmux because the module pads are not connected until the pinmux is
configured.
Signed-off-by: Stephanos Ioannidis <root@stephanos.io>
547a5e9eb0