Doc: DT: mfd: ams: add DT documentation for as3722

Doc: DT: mfd: ams: add DT documentation for as3722

Add Devicetree binding documentation for as3722
from upstream.

This is based on mainline Commit
https://patchwork.kernel.org/patch/2975721/
Contributed by:
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Florian Lobmaier <florian.lobmaier@ams.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Change-Id: Ie8a17d9aaad883ef59f928f389ac6b6233bbaa6f
Signed-off-by: Bibek Basu <bbasu@nvidia.com>
Reviewed-on: http://git-master/r/405114
(cherry picked from commit 32a858069d16069e4fa71c684b83ca054f7c7e23)
Reviewed-on: http://git-master/r/406388
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Laxman Dewangan <ldewangan@nvidia.com>