Previously, TF-A could not support large RSA key sizes as the
configuration options passed to MBEDTLS prevented storing and performing
calculations with the larger, higher-precision numbers required. With
these changes to the arguments passed to MBEDTLS, TF-A now supports
using 3072 (3K) and 4096 (4K) keys in certificates.

Change-Id: Ib73a6773145d2faa25c28d04f9a42e86f2fd555f
Signed-off-by: Justin Chadwell <justin.chadwell@arm.com>