ARM: dts: apalis/colibri-imx6/6ull/7: proper gpio-key node names

The node names should be generic and DT schema expects certain pattern
(e.g. starting with key/button/switch).

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>