rtc: isl12057: enable support for the standard "wakeup-source" property

Though the isl12057 rtc driver should and will continue to support the
legacy "isil,irq2-can-wakeup-machine" property to enable RTC as the
wakeup source, we need to add support for the new standard property
"wakeup-source".

This patch adds support for "wakeup-source" property in addition to the
existing "isil,irq2-can-wakeup-machine" property.

Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Cc: rtc-linux@googlegroups.com
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>