[LIBS] libfdt: Fix libfdt_parse_fileinfo() API

Currently, libfdt_parse_fileinfo() API assume that FDT strings will
immediately follow FDT data. This works is most of the case but if
bootloader (such as u-boot) modify FDT before passing then it will
fail.

The correct way of determining location of FDT strings and FDT data
in FDT blob is to use offset_xxx fields of FDT header.

Signed-off-by: Anup Patel <anup@brainfault.org>