tools/gpio: Don't use u_int32_t

u_int32_t is a non-standard version of uint32_t, that was apparently
introduced by BSD. Use uint32_t from stdint.h instead.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>