firmware: imx: sm-misc: Make scmi_imx_misc_ctrl_nb variable static

firmware: imx: sm-misc: Make scmi_imx_misc_ctrl_nb variable static

File-scope 'scmi_imx_misc_ctrl_nb' is not used outside of this unit, so
make it static to silence sparse warning:

  sm-misc.c:19:23: warning: symbol 'scmi_imx_misc_ctrl_nb' was not declared. Should it be static?

Reviewed-by: Daniel Baluta <daniel.baluta@nxp.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Frank Li <Frank.Li@nxp.com>