[modtrezorio] Add GPT support for sd-protect feature

[brianddk]

The modtrezorio code appears to have stubbed out references to GPT support, but appears incomplete. Currently the find_volume function only supports MBR partition searching. It would be useful to add GPT.

With many of the macOS and Linux distributions supporting GPT partition on USB, it seems a logical choice to support both. In the event of blank SD cards, MBR could still be the default, but adding GPT support would be useful.