riscv: add support for hugepage migration

Generic arch_hugetlb_migration_supported can provide appropriate
support for hugepage migration when enable ARCH_ENABLE_HUGEPAGE_MIGRATION,
so enable it for hugepage migration in riscv.

Signed-off-by: Chen Wandun <chenwandun@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>