Documentation: tpm: Add TPM security docs toctree entry

Stephen Rothwell reports htmldocs warning when merging tpmdd tree for
linux-next:

Documentation/security/tpm/tpm-security.rst: WARNING: document isn't included in any toctree

Add toctree entry for TPM security docs to fix above warning.

Fixes: ddfb3687c538 ("Documentation: add tpm-security.rst")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/linux-next/20240506162105.42ce2ff7@canb.auug.org.au/
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>