dt-bindings: imx-gpc: correct the DOMAIN_INDEX using

Actually DOMAIN_INDEX is not used by the client devices to refer to
the power domain, it uses phandle. Corrent the binding doc a bit
to avoid confusing.

Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>