[mmio] fix ptr types to avoid arithmetic warnings
[mmio] fix ptr types to avoid arithmetic warnings

In mmio_region_memcpy32() the pointer buf, is operated on as a byte
sized pointer, and shouldn't be specified as a void ptr.

Signed-off-by: Drew Macrae <32d48ac45113625c0a04afe7343eae478a303684@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan