KEYS: trusted: improve scalability of trust source config

Enabling trusted keys requires at least one trust source implementation
(currently TPM, TEE or CAAM) to be enabled. Currently, this is
done by checking each trust source's config option individually.
This does not scale when more trust sources like the one for DCP
are added, because the condition will get long and hard to read.

Add config HAVE_TRUSTED_KEYS which is set to true by each trust source
once its enabled and adapt the check for having at least one active trust
source to use this option. Whenever a new trust source is added, it now
needs to select HAVE_TRUSTED_KEYS.

Signed-off-by: David Gstir <david@sigma-star.at>
Tested-by: Jarkko Sakkinen <jarkko@kernel.org> # for TRUSTED_KEYS_TPM
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>