Copy include/linux/limits.h from Linux 5.1-rc1
(removing #include <uapi/linux/limits.h>).

While we are here, add SPDX License tag to <linux/kernel.h>.

Based on the following Linux commits:

 - 54d50897d544 ("linux/kernel.h: split *_MAX and *_MIN macros
   into <linux/limits.h>")

 - 2dc0e68d5ada ("linux/kernel.h: use 'short' to define USHRT_MAX,
   SHRT_MAX, SHRT_MIN")

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>