Replaced the legacy binding "gpio-key,wakeup" with "wakeup-source" as
noted in the kernel documentation.

Signed-off-by: Teresa Remmet <t.remmet@phytec.de>
Reported-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>