cert_create: Use RSASSA-PSS signature scheme for certificates

This patch modifies the `cert_create` tool to use RSASSA-PSS scheme for
signing the certificates. This is compliant with RSA PKCS_2_1 standard as
mandated by TBBR.

Note that the certificates generated by using cert_create tool after this
patch can be authenticated during TBB only if the corresponding mbedtls
driver in ARM Trusted Firmware has the corresponding support.

Change-Id: If224f41c76b3c4765ae2af5259e67f73602818a4
Signed-off-by: Soby Mathew <soby.mathew@arm.com>