Add PKCS #5 v2.0 key derivation function 2 (PBKDF2)

This commit implements another crypto extension to support the
key derivation function defined in PKCS #5 v2.0, republished as RFC 2898
(https://www.ietf.org/rfc/rfc2898.txt section 5.2).
The underlying pseudorandom function is HMAC-SHA1, as recommended. However,
it would be trivial to support the other HMAC functions already implemented
in OP-TEE.

See documentation/extensions/crypto_pbkdf2.md for details.

Tested on PLATFORM=vexpress-qemu_virt with the test vectors from RFC 6070
(https://www.ietf.org/rfc/rfc6070.txt).

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Signed-off-by: Xiaoqiang Du <xiaoqiang.du@linaro.org>