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>

soc/mediatek/mt8xxx/mtk_adsp_load.py

@@ -277,7 +277,13 @@ def find_winstream(maps):
     magic = b'\x74\x5f\x6a\xd0\x79\xe2\x4f\x00\xcd\xb8\xbd\xf9'
     for m in maps:
         if "dram" in m:
-            magoff = maps[m].find(magic)
+            # Some python versions produce bus errors (!) on the
+            # hardware when finding a 12 byte substring (maybe a SIMD
+            # load that the hardware doesn't like?).  Do it in two
+            # chunks.
+            magoff = maps[m].find(magic[0:8])
+            if magoff >= 0:
+                magoff = maps[m].find(magic[8:], magoff) - 8
             if magoff >= 0:
                 addr = le4(maps[m][magoff + 12 : magoff + 16])
                 return addr