soc/mediatek/mtk_adsp: Use smaller accesses when find()ing in device memory

Recent Python interpreters have started tossing bus errors from this
12-byte string search (the loader is looking for the winstream
descriptor in the live firmware image).  My guess is that there's a
SIMD optimization that's been added that's trying to do e.g. a 16 byte
load, and something in the fabric is kicking that out.  Note that this
is 100% a software change: the same hardware with one version of the
host environment works, and an update breaks it.

But really I have no idea what's happening here, the memory region in
question is documented as system DRAM, the same bus regular process
memory is on (it's just not kernel-utilized memory).  All I know is
that the bus error is introduced with a Python upgrade from 3.8.20 to
3.11.10.

Regardless, it's no great hardship to do the search on 64 bit chunks.

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