Since 253fb33 (input: gpio-keys: convert to input framework) the
gpio-buttons are registered with the input framework which has the
side effect that they are activated during boot and no longer have
to be activated manually by activating the input device console.
This reveals that the gpio-button polarities are wrong: The autoboot
is no longer running through since a gpio button press is wrongly
detected.
Fix the polarities in the barebox dts for now to get back a working
board. A proper fix has been sent upstream to the kernel. Once this
has landed and propagated back to barebox this patch can be reverted.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>