imx: imx6: Add comment to gpr_init function

This function can be used only for some of the nxp SoC. Make
it explicit in the comment. This adjust a bit commit
3aa4b703

 ("imx: imx6: Move gpr_init() function to soc.c")

Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>
Acked-by: Jagan Teki <jagan@amarulasolutions.com>