regulator: dt: add policy to have property "regulator-compatible"

Add the policy for regulator DT such that if device have multiple
regulator and its binding contains a child node that describes each
regulator then each regulator child node must have the property
"regulator-compatible" which matches with regulator name of their
hardware counterparts.
Modify the DT documentation of regulator devices to reflect this
policy.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>