ARM: dts: imx53-qsb: Fix gpio button polarity

The polarity of the gpio buttons is defined to '0' which is
active high. The buttons are active low though which has been verified by
testing it and by looking into the schematics. While at it use
defines rathers than numbers for the key codes.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>