arch: arm: Fix MMU disable function

Use dsb barrier before disabling the mmu to ensure all memory
accesses have completed. Also as per arm reference manual use
isb & dsb barriers for tlb invalidate & mmu enable, disable
functions.

CRs-Fixed: 514600
Change-Id: I7f9a041189cbf87c9b41ebefba07893b2798f037
3 files changed