arm64_mmu: Add data synchronization barrier after page tables are written

The page tables must be committed to system memory before we can proceed
enabling the MMU. ISB() is not enough to do this.

arch/arm64/src/common/arm64_mmu.c

@@ -536,6 +536,7 @@ static void enable_mmu_el1(unsigned int flags)
 
   /* Ensure these changes are seen before MMU is enabled */
 
+  ARM64_DSB();
   ARM64_ISB();
 
   /* Enable the MMU and data cache */