Though the driver will continue to check for and support the legacy
"isil,irq2-can-wakeup-machine" boolean property to wakeup source,
"wakeup-source" is the new standard binding.

This patch replaces the legacy "isil,irq2-can-wakeup-machine" with the
unified "wakeup-source" property in order to avoid any futher copy-paste
duplication.

Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Gregory Clement <gregory.clement@free-electrons.com>
Cc: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>