!469 打开after_mmu_enable之后被注释的disable ttbr0操作

Merge pull request !469 from jesven/fix-after_mmu_enable

libcpu/aarch64/cortex-a/entry_point.S

@@ -142,13 +142,11 @@ __start:
     ret
 
 after_mmu_enable:
-#if 0
     mrs x0, tcr_el1          /* disable ttbr0, only using kernel space */
     orr x0, x0, #(1 << 7)
     msr tcr_el1, x0
     msr ttbr0_el1, xzr
     dsb sy
-#endif
 
     mov     x0, #1
     msr     spsel, x0