Fix MIPS cache operations and enable HAVE_CPU_CACHE_ALIGN on MIPS

- The range-based cache operations on MIPS were broken and only worked
  properly when BOTH the address and size were multiples of the cache
  line size. If this was not the case, the last cache line of the range
  would not be touched!

  Fix is to align start/end pointers to cache lines before iterating.

- To my knowledge all MIPS processors have a cache, so I enabled
  HAVE_CPU_CACHE_ALIGN by default. This also allows mmu-mips.c to use
  the CACHEALIGN_UP/DOWN macros.

- Make jz4760/system-target.h define its cache line size properly.

Change-Id: I1fcd04a59791daa233b9699f04d5ac1cc6bacee7

firmware/export/system.h

@@ -253,6 +253,10 @@ static inline void cpu_boost_unlock(void)
     #define MIN_STACK_ALIGN 8
 #endif
 
+#ifdef CPU_MIPS
+    #define HAVE_CPU_CACHE_ALIGN
+#endif
+
 /* Define this if target has support for generating backtraces */
 #ifdef CPU_ARM
     #define HAVE_RB_BACKTRACE