sim: Initialize PSA Crypto subsystem based on psa-crypto-api feature

If the psa-crypto-api feature is defined, the simulator will
initialize the PSA Crypto API exactly once. It needs also to
enable the test external RNG as the assumption is that the
PSA subsystem is configured to use MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG

Signed-off-by: David Brown <david.brown@linaro.org>
Signed-off-by: Antonio de Angelis <Antonio.deAngelis@arm.com>
Signed-off-by: Matthew Dalzell <matthew.dalzell@arm.com>
Change-Id: Id02727b8673867ecf1e4fbbdfa3c4b6d6f98f8df