KEYS: asym_tpm: Implement signature verification [ver #2]

This patch implements the verify_signature operation.  The public key
portion extracted from the TPM key blob is used.  The operation is
performed entirely in software using the crypto API.

Signed-off-by: Denis Kenzior <denkenz@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Marcel Holtmann <marcel@holtmann.org>
Reviewed-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: James Morris <james.morris@microsoft.com>