This commit adds the asynchronous UART API testing support on the SAM
E54 Xplained Pro board.
The SERCOM1 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 SERCOM1 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>
974b44e9e5