of/fdt: update of_get_flat_dt_prop in prep for libfdt

of/fdt: update of_get_flat_dt_prop in prep for libfdt

Make of_get_flat_dt_prop arguments compatible with libfdt fdt_getprop
call in preparation to convert FDT code to use libfdt. Make the return
value const and the property length ptr type an int.

Signed-off-by: Rob Herring <robh@kernel.org>
Tested-by: Michal Simek <michal.simek@xilinx.com>
Tested-by: Grant Likely <grant.likely@linaro.org>
Tested-by: Stephen Chivers <schivers@csc.com>
(cherry picked from commit 9d0c4dfedd96ee54fc075b16d02f82499c8cc3a6)
Signed-off-by: Mark Brown <broonie@linaro.org>
Conflicts:
	arch/arc/kernel/devtree.c
	arch/arm/kernel/devtree.c
	arch/arm/mach-exynos/exynos.c
	arch/arm/plat-samsung/s5p-dev-mfc.c
	arch/powerpc/kernel/epapr_paravirt.c
	arch/powerpc/kernel/prom.c
	arch/powerpc/mm/hash_utils_64.c
	arch/powerpc/platforms/powernv/opal.c
	arch/xtensa/kernel/setup.c
	drivers/of/fdt.c