ARM: dts: imx6sx: correct property name for wakeup source

Commit(def56bba input: snvs_pwrkey: use "wakeup-source"
as deivce tree property name) replaces the property name
of "wakeup" with "wakeup-source", update this change
in i.MX6SX dtsi accordingly.

Signed-off-by: Anson Huang <b20788@freescale.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>