ecp_curves: Minor rework for p384

This patch adjusts formatting, documentation and testing.

Signed-off-by: Minos Galanakis <minos.galanakis@arm.com>
diff --git a/tests/suites/test_suite_ecp.function b/tests/suites/test_suite_ecp.function
index 440a1e3..78aca8a 100644
--- a/tests/suites/test_suite_ecp.function
+++ b/tests/suites/test_suite_ecp.function
@@ -1452,7 +1452,7 @@
     size_t limbs = limbs_N;
     size_t bytes = limbs * sizeof(mbedtls_mpi_uint);
 
-    TEST_EQUAL(limbs_X, (766 / biL) + 1);
+    TEST_EQUAL(limbs_X, 2 * limbs);
     TEST_EQUAL(limbs_res, limbs);
 
     TEST_EQUAL(mbedtls_mpi_mod_modulus_setup(