bn_mul.h: require at least ARMv6 to enable the ARM DSP code

Commit 16b1bd8 "bn_mul.h: add ARM DSP optimized MULADDC code"
added some ARM DSP instructions that was assumed to always be available
when __ARM_FEATURE_DSP is defined to 1. Unfortunately it appears that
the ARMv5TE architecture (GCC flag -march=armv5te) supports the DSP
instructions, but only in Thumb mode and not in ARM mode, despite
defining __ARM_FEATURE_DSP in both cases.

This patch fixes the build issue by requiring at least ARMv6 in addition
to the DSP feature.

include/mbedtls/bn_mul.h

@@ -642,7 +642,8 @@
            "r6", "r7", "r8", "r9", "cc"         \
          );
 
-#elif defined (__ARM_FEATURE_DSP) && (__ARM_FEATURE_DSP == 1)
+#elif (__ARM_ARCH >= 6) && \
+    defined (__ARM_FEATURE_DSP) && (__ARM_FEATURE_DSP == 1)
 
 #define MULADDC_INIT                            \
     asm(