regulator: Document binding for regulator suspend state for PM state

This patch add regulator suspend state to constraint in dt file. The regulation_
constraints structure already has regulator suspend state field as following.
The regulator suspend state control the state of regulator according to
PM (Power Management) state.
- struct regulator_state state_disk
- struct regulator_state state_mem

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>