ARM: decompressor: Flush tlb before swiching domain 0 to client mode

If the bootloader used a page table that is incompatible with domain 0
in client mode, and boots with the mmu on, then swithing domain 0 to
client mode causes a fault if we don't flush the tlb after updating
the page table pointer.

v2: Add ISB before loading dacr.

Signed-off-by: Arve Hjønnevåg <arve@android.com>