fdt: Add INT32_MAX to kernel.h for libfdt

Unfortunately libfdt needs this value now, which is present in the
stdint.h header. That file is just a placeholder in U-Boot and these sorts
of constants appear in the linux/kernel.h header instead.

To keep libfdt happy, add INT32_MAX too.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>