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 b0ebee4..d7fe6b9 100644
--- a/tests/suites/test_suite_mpi.function
+++ b/tests/suites/test_suite_mpi.function
@@ -872,7 +872,7 @@
 }
 /* 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, char *input_X, int div_result )