tests: mcumgr: exclude lpcxpresso51u68

The test is failing for lpcxpresso51u68 with:

soc_flash_lpc.c:25:2: error: #error No matching compatible for
soc_flash_lpc.c
   25 | #error No matching compatible for soc_flash_lpc.c
      |  ^~~~~

Add the board to the exclude list, sort the list as well.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>