ARM: dts: imx: replace legacy wakeup property with 'wakeup-source'

Though the keyboard and other driver will continue to support the legacy
"gpio-key,wakeup", "linux,wakeup" and "enable-sdio-wakeup" boolean
property to enable the wakeup source, "wakeup-source" is the new
standard binding.

This patch replaces all the legacy wakeup properties with the unified
"wakeup-source" property in order to avoid any futher copy-paste
duplication.

Cc: Sascha Hauer <kernel@pengutronix.de>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>