tpm: use more than sha256 on pcr_extend

The current tpm2_pcr_extend is hardcoded using SHA256.
Let's make the actual command to the TPM2 configurable so we can support
a wider range of algorithms and keep the current command line as-is i.e
limited to SHA256 only

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>