Move dependancy to specific test cases

Move dependancy on `MBEDTLS_MPI_MAX_BITS` to apply to the specific test cases
which will break when `MBEDTLS_MPI_MAX_BITS` is too small. This re-enables
previous tests that were turned off accidentally.

Signed-off-by: Chris Jones <christopher.jones@arm.com>
diff --git a/tests/suites/test_suite_mpi.function b/tests/suites/test_suite_mpi.function
index f44db70..a16e4db 100644
--- a/tests/suites/test_suite_mpi.function
+++ b/tests/suites/test_suite_mpi.function
@@ -1136,7 +1136,7 @@
 }
 /* END_CASE */
 
-/* BEGIN_CASE depends_on:MPI_MAX_BITS_LARGER_THAN_256 */
+/* BEGIN_CASE */
 void mbedtls_mpi_exp_mod( int radix_A, char * input_A, int radix_E,
                           char * input_E, int radix_N, char * input_N,
                           int radix_RR, char * input_RR, int radix_X,