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>
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Git-commit: 9d0c4dfedd96ee54fc075b16d02f82499c8cc3a6
[joonwoop@codeaurora.org: updated drivers/base/dma-contiguous.c to use 'const'
 qualifier.  dropped arch/arm/mach-exynos/exynos.c.]
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>