ARM: dts: am335x/am437x: remove unneeded unit name for gpio-keys nodes

This patch fixes the following DTC warnings for many boards:

"Node /gpio_keys/button0@10 has a unit name, but no reg property"

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>