The upcoming v1.5.1 version of libfdt adds a dependency on the C99
INT32_MAX symbol.
This is ok for use in the host tool, but libfdt can also be embedded
into barebox, where it would fail as we don't define this anywhere.

Fix this by providing an appropriate definition.

Reported-by: Rouven Czerwinski <r.czerwinski@pengutronix.de>
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>