tests/boards/mtk_adsp: Handle protocol skew

Recent kernel drivers for some devices have swapped the mailbox device
used for replies.  It used to be that all commands in either direction
were sent on mbox0 and all replies on mbox1.  Now mbox0 handles
commands and replies for "DSP to Host" communication, and mbox1 is for
"Host to DSP".  Listen to both devices for our simple test.

Signed-off-by: Andy Ross <andyross@google.com>