ARM: dts: at91: replace gpio-key,wakeup with wakeup-source for sam9260ek

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.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>