riscv: Enable HAVE_ARCH_HUGE_VMAP for 64BIT

This sets the HAVE_ARCH_HUGE_VMAP option, and defines the required page
table functions. With this feature, ioremap area will be mapped with
huge page granularity according to its actual size. This feature can be
disabled by kernel parameter "nohugeiomap".

Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Reviewed-by: Björn Töpel <bjorn@kernel.org>
Tested-by: Björn Töpel <bjorn@kernel.org>
Link: https://lore.kernel.org/r/20221012120038.1034354-2-liushixin2@huawei.com
[Palmer: minor formatting]
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>