Make it clear that this is an abstraction of the random generator
abstraction, and not an abstraction of the PSA random generator.
mbedtls_psa_get_random and MBEDTLS_PSA_RANDOM_STATE are public-facing
definitions and will be moved in a subsequent commit.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>