regulator: i.MX anatop: Allow supply regulator

The anatop regulators are SoC internal LDO regulators usually supplied
by an external PMIC. This patch adds support for specifying the supply
from the device tree using the vin-supply property.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>