docs: document DCP-backed trusted keys kernel params

Document the kernel parameters trusted.dcp_use_otp_key
and trusted.dcp_skip_zk_test for DCP-backed trusted keys.

Co-developed-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Richard Weinberger <richard@nod.at>
Co-developed-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>
Signed-off-by: David Gstir <david@sigma-star.at>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>