dt-bindings: power: Add power-domains-child-ids property

Add binding documentation for the new power-domains-child-ids property,
which works in conjunction with the existing power-domains property to
establish parent-child relationships between a multi-domain power domain
provider and external parent domains.

Each element in the uint32 array identifies the child domain
ID (index) within the provider that should be made a child domain of
the corresponding phandle entry in power-domains. The two arrays must
have the same number of elements.

Signed-off-by: Kevin Hilman (TI) <khilman@baylibre.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>