ARM: ux500: Rename gpio_keys in the Device Tree file

Ensure each button is individually identifiable by using
their correct unique given device names.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>