soc/mt8196: Add interrupt routing support

The MT8196 device has a newer interrupt controller that acts like the
legacy ones once initialized (see intc_mtk_adsp.c).  But it has some
(only slightly) more complicated routing control that must be
initialized on reset, as the default is "don't deliver any interrupts
at all".  Previous versions of the device integration worked becuase
they relied on a SOF binary to be loaded at boot, but obviously that
doesn't work for a Zephyr-based SOF firmware image.

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