regulator: add binding for the SY8106A voltage regulator

SY8106A is an I2C-controlled adjustable voltage regulator made by
Silergy Corp.

Add its device tree binding.

Signed-off-by: Ondrej Jirman <megous@megous.com>
[Icenowy: Change commit message and slight fixes]
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>