Though the keyboard driver for GPIO buttons(gpio-keys) will continue to
check for/support the legacy "gpio-key,wakeup" boolean property to
enable gpio buttons as wakeup source, "wakeup-source" is the new
standard binding.

This patch replaces the legacy "gpio-key,wakeup" with the unified
"wakeup-source" property in order to avoid any further copy-paste
duplication.

Cc: "Benoît Cousson" <bcousson@baylibre.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>