arm64: mm: Add THP TLB entries to general mmu_gather

When arm64 moved over to the core mmu_gather, it lost the logic to
flush THP TLB entries (tlb_remove_pmd_tlb_entry was removed and the
core implementation only signals that the mmu_gather needs a flush).

This patch ensures that tlb_add_flush is called for THP TLB entries.

Signed-off-by: Steve Capper <steve.capper@linaro.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>