dt-bindings: add multidomain support to i.MX GPC DT binding

This adds a new binding for the Freescale i.MX GPC block, which allows
to describe multiple power domains in a more natural way. The driver
will continue to support the old binding for existing DTBs, but new
features like the additional domains present on i.MX6SX will only be
usable with the new binding.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>