dt-bindings: power: add defines for i.MX8MM power domains

Adding defines for i.MX8MM GPC power domains.

Tested-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>