riscv/mm: Enable THP migration

Add two THP helpers required to create PMD migration swap entries,
and enable THP migration via ARCH_ENABLE_THP_MIGRATION. This can
reduce time of THP migration without splitting and guarantee the
migrated pages are still contiguous.

Signed-off-by: Nanyong Sun <sunnanyong@huawei.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>