bn_mul.h: add ARM DSP optimized MULADDC code

The Cortex M4, M7 MCUs and the Cortex A CPUs support the ARM DSP
instructions, and especially the umaal instruction which greatly
speed up MULADDC code. In addition the patch switched the ASM
constraints to registers instead of memory, giving the opportunity
for the compiler to load them the best way.

The speed improvement is variable depending on the crypto operation
and the CPU. Here are the results on a Cortex M4, a Cortex M7 and a
Cortex A8. All tests have been done with GCC 6.3 using -O2. RSA uses a
RSA-4096 key. ECDSA uses a secp256r1 curve EC key pair.

                 +--------+--------+--------+
                 |   M4   |   M7   |   A8   |
+----------------+--------+--------+--------+
| ECDSA signing  |  +6.3% |  +7.9% |  +4.1% |
+----------------+--------+--------+--------+
| RSA signing    | +43.7% | +68.3% | +26.3% |
+----------------+--------+--------+--------+
| RSA encryption |  +3.4% |  +9.7% |  +3.6% |
+----------------+--------+--------+--------+
| RSA decryption | +43.0% | +67.8% | +22.8% |
+----------------+--------+--------+--------+

I ran the whole testsuite on the Cortex A8 Linux environment, and it
all passes.

include/mbedtls/bn_mul.h

@@ -630,6 +630,23 @@
            "r6", "r7", "r8", "r9", "cc"         \
          );
 
+#elif defined(__ARM_FEATURE_DSP)
+
+#define MULADDC_INIT                            \
+    asm(
+
+#define MULADDC_CORE                            \
+            "ldr    r0, [%0], #4        \n\t"   \
+            "ldr    r1, [%1]            \n\t"   \
+            "umaal  r1, %2, %3, r0      \n\t"   \
+            "str    r1, [%1], #4        \n\t"
+
+#define MULADDC_STOP                            \
+         : "=r" (s),  "=r" (d), "=r" (c)        \
+         : "r" (b), "0" (s), "1" (d), "2" (c)   \
+         : "r0", "r1", "memory"                 \
+         );
+
 #else
 
 #define MULADDC_INIT                                    \