Fix MMCAU_SHA256 type warnings

Fix warnings in the usage of MMCAU_SHA256 routines, where digest is
expected to be `uint32_t*`, but is defined as `word32*`, which results
in:

```
expected 'uint32_t *' {aka 'long unsigned int *'} but argument is of
type 'word32 *' {aka 'unsigned int *'}
```

Signed-off-by: Fabio Utzig <utzig@apache.org>