The keyboard driver for GPIO buttons(gpio-keys) checks for one of the
two boolean properties to enable gpio buttons as wakeup source:
1. "wakeup-source" or
2. the legacy "gpio-key,wakeup"

However juno, ste-snowball and emev2-kzm9d dts file have a undetected
"wakeup" property to indictate the wakeup source.

This patch fixes it by making use of "wakeup-source" property.

Cc: Magnus Damm <magnus.damm@gmail.com>
Acked-by: Simon Horman <horms@verge.net.au>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Olof Johansson <olof@lixom.net>