arm/arm64: Invalidate TLB before enabling MMU

Let's invalidate the TLB before enabling the MMU, not after, so we don't
accidently use a stale TLB mapping. For arm, we add a TLBIALL operation,
which applies only to the PE that executed the instruction [1]. For arm64,
we already do that in asm_mmu_enable.

We now find ourselves in a situation where we issue an extra invalidation
after asm_mmu_enable returns. Remove this redundant call to tlb_flush_all.

[1] ARM DDI 0406C.d, section B3.10.6

Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>