sandbox: tpm: Support extending a PCR multiple times

It is fairly easy to handle this case and it makes the emulator more
useful, since PCRs are commonly extended several times.

Add support for this, using U-Boot's sha256 support.

For now sandbox only supports a single PCR, but that is enough for the
tests that currently exist.

Signed-off-by: Simon Glass <sjg@chromium.org>