Documentation: arm64 also supports disable hugeiomap

arm64 also supports disable hugeiomap,updated documentation.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Link: https://lore.kernel.org/r/1599740386-47210-1-git-send-email-tiantao6@hisilicon.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>