regulator: add document bindings for sy8827n

Add device tree binding information for sy8827n regulator driver.

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Link: https://lore.kernel.org/r/20200702171335.59f5e79b@xhacker.debian
Signed-off-by: Mark Brown <broonie@kernel.org>