regulator: slg51000: add slg51000 regulator driver

Adding regulator driver for the device Dialog SLG51000.

The SLG51000 device contains seven compact and customizable low
dropout regulators and is designed for high performance camera modules
and other small multi-rail applications.

Signed-off-by: Eric Jeong <eric.jeong.opensource@diasemi.com>
Signed-off-by: Mark Brown <broonie@kernel.org>