doc: device-tree-bindings: regulator: anatop regulator

Document the bindings for fsl,anatop-regulator

Signed-off-by: Ying-Chun Liu (PaulLiu) <paul.liu@linaro.org>
Reviewed-by: Sean Anderson <sean.anderson@seco.com>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Cc: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>