dts: Cleanup warnings associated with gpio_keys that aren't buttons

We get several warnings of the form:

 	Warning (unit_address_vs_reg): /gpio_keys/joystick@0:
	node has a unit name, but no reg property

Fix by dropping the unit address and renaming the node names to include
more information about the gpio_key.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>