firmware: imx: add pm svc headfile

Add SCU PM SVC related protocol definitions which will be used by
a number of PM functions like Power Domain, Clock, Reset and etc.
The detailed implementation of each function will put in the individual
function drivers.

Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <kernel@pengutronix.de>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>