tpm: Export the open/close functions

tpm: Export the open/close functions

At present these functions are not accessible outside the TPM library, but
in some cases we need to call them. Export them in the header file and add
a define for the SHA1 digest size.

Also adjust tpm_open() to call tpm_close() first so that the TPM is in a
known state before opening (e.g. by a previous phase of U-Boot).

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