ARM: dts: at91: replace gpio-key,wakeup with wakeup-source property

Most of the legacy "gpio-key,wakeup" property are already replaced
with "wakeup-source". However few occurrences of old property has
popped up again, probably from the remnants in downstream trees.
Almost all of those were remove couple of years back.

Replace the legacy properties with the unified "wakeup-source"
property introduced in the commit 700a38b2 ("Input: gpio_keys -
switch to using generic device properties")

Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Peter Rosin <peda@axentia.se>
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>