soc/mediatek/adsp: Union mbox ISRs

The SOF source code is confusing.  On some hardware these devices have
distinct IRQs assigned, and on others they seem to share an ISR for
all.  Leave the existing assignments in place for SOF-compatibility,
but union all the devices into a single ISR path that will poll each
(there are only two).  This will work in all configurations, and we
can figure out the proper architecture at leisure.

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