regulator: anatop: patching to device-tree property "reg".

Change "reg" to "anatop-reg-offset" due to there is a warning of handling no
size field in reg.

This patch also adds the missing device-tree binding documentation.

Signed-off-by: Ying-Chun Liu (PaulLiu) <paul.liu@linaro.org>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>