tpm: Implement firmware API call get-maximum-cmd-size

Implement the firmware API call get-maximum-cmd-size that returns a TPM's
maximum command size. The caller should use the size returned by this
function to allocate a buffer large enough for TPM commands and also
responses.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>