Update documentation for Trusted and Encrypted Keys with TEE as a new
trust source. Following is brief description of updates:

- Add a section to demonstrate a list of supported devices along with
  their security properties/guarantees.
- Add a key generation section.
- Updates for usage section including differences specific to a trust
  source.
Co-developed-by: Elaine Palmer <erpalmer@us.ibm.com>
Signed-off-by: Elaine Palmer <erpalmer@us.ibm.com>
Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>