tpm: Add NULL primary creation

The session handling code uses a "salted" session, meaning a session
whose salt is encrypted to the public part of another TPM key so an
observer cannot obtain it (and thus deduce the session keys).  This
patch creates and context saves in the tpm_chip area the primary key
of the NULL hierarchy for this purpose.

[jarkko@kernel.org: fixed documentation errors]
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Tested-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>