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
3aa4b703b483f165dd ("imx: imx6: Move gpr_init() function to soc.c")

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

arch/arm/mach-imx/mx6/soc.c

@@ -651,6 +651,11 @@ void imx_setup_hdmi(void)
 }
 #endif
 
+
+/*
+ * gpr_init() function is common for boards using MX6S, MX6DL, MX6D,
+ * MX6Q and MX6QP processors
+ */
 void gpr_init(void)
 {
 	struct iomuxc *iomux = (struct iomuxc *)IOMUXC_BASE_ADDR;