arm: cstart64.S: Downgrade TLBI to non-shareable in asm_mmu_enable

There's really no need to invalidate the TLB entries for all CPUs when
enabling the MMU for the current CPU, so use the non-shareable version of
the TLBI operation (and downgrade the DSB accordingly).

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