of/fdt: introduce of_get_flat_dt_size

Add a wrapper function to retrieve the FDT size from the FDT header. This
is primarily to avoid libfdt include paths for the whole kernel.

Signed-off-by: Rob Herring <robh@kernel.org>
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: c0556d3f2c3f42eaed049139ce6f0899ecdb0217
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>