regulator: add binding for the SY8824C voltage regulator

SY8824C is an I2C-controlled adjustable voltage regulator made by
Silergy Corp.

Add its device tree binding.

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Link: https://lore.kernel.org/r/20190827163341.61df63a7@xhacker.debian


Signed-off-by: Mark Brown <broonie@kernel.org>