ARM: dts: imx6qdl-gw59{10,13}: fix user pushbutton GPIO offset

The GW5910 and GW5913 have a user pushbutton that is tied to the
Gateworks System Controller GPIO offset 2. Fix the invalid offset of 0.

Fixes: 64bf0a0a

 ("ARM: dts: imx6qdl-gw: add Gateworks System Controller support")
Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>