ARM: imx: use __initconst for const init definition

0-DAY kernel build testing backend reports the following.

 scripts/checkpatch.pl 0001-ARM-imx-add-support-code-for-IMX50-based-machines.patch
 # many are suggestions rather than must-fix

 ERROR: Use of const init definition must use __initconst
 #80: arch/arm/mach-imx/mach-imx50.c:26:
 +static const char *imx50_dt_board_compat[] __initdata = {

While at it, fix the error globally for IMX platform.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Acked-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>