tpm: Rename TPM2_OA_TMPL to TPM2_OA_NULL_KEY and make it local

Rename and document TPM2_OA_TMPL, as originally requested in the patch
set review, but left unaddressed without any appropriate reasoning. The
new name is TPM2_OA_NULL_KEY, has a documentation and is local only to
tpm2-sessions.c.

Link: https://lore.kernel.org/linux-integrity/ddbeb8111f48a8ddb0b8fca248dff6cc9d7079b2.camel@HansenPartnership.com/
Link: https://lore.kernel.org/linux-integrity/CZCKTWU6ZCC9.2UTEQPEVICYHL@suppilovahvero/


Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>