libfdt: reduce the number of headers included from libfdt_env.h

Currently, libfdt_env.h includes <linux/kernel.h> just for INT_MAX.

<linux/kernel.h> pulls in a lots of broat.

Thanks to commit 54d50897 ("linux/kernel.h: split *_MAX and *_MIN
macros into <linux/limits.h>"), <linux/kernel.h> can be replaced with
<linux/limits.h>.

This saves including dozens of headers.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Rob Herring <robh@kernel.org>