feat(lib/psa): add measured boot API

A secure enclave could provide an alternate
backend for measured boot. This API can be used
to store measurements in a secure enclave, which
provides the measured boot runtime service.

Signed-off-by: Tamas Ban <tamas.ban@arm.com>
Change-Id: I2448e324e7ece6b318403c5937dfe7abea53d0f3