regulator: add support for SY8827N regulator

The SY8827N from Silergy Corp is a single output DC/DC converter. The
voltage can be controlled via I2C.

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Link: https://lore.kernel.org/r/20200702171438.20edc523@xhacker.debian
Signed-off-by: Mark Brown <broonie@kernel.org>