feat(lib/psa): add read_measurement API

This API is added for testing purposes. It makes possible to write test
cases that read measurements back after extending them, and compare
them to expected results.

Change-Id: Iec447d972fdd54a56ab933a065476e0f4d35a6fc
Signed-off-by: Mate Toth-Pal <mate.toth-pal@arm.com>