doc: trusted-encrypted: describe new CAAM trust source

Update documentation for trusted key use with the Cryptographic
Acceleration and Assurance Module (CAAM), an IP on NXP SoCs.

Reviewed-by: Pankaj Gupta <pankaj.gupta@nxp.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>