regulator: ab3100: device tree support

This implements device tree support for the AB3100 regulators
driver. The initial settings are moved out of platform data
and into the driver for the device tree case, as it appears
that there is no way to supply this as AUXDATA for an I2C
device. The style and bindings are heavily inspired by
Lee Jones' style for AB8500.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>