boards/mediatek: Add mt8196_adsp

Add Zephyr support for the Audio DSP on the MT8196 SOC.  This is a
very similar device to previous designs.  Most of this patch is just
DTS.

The biggest delta is the more complicated second level interrupt
controller, though it is still able to be represented using some
vaguely clever DTS config over the older intc_mtk_adsp driver.

Also the memory layout is slightly different, requiring a little
indirection to set the initial boot stack address and log output
buffer.  And the timer "irq_ack" register bits moved.

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