input: snvs_pwrkey: use "wakeup-source" as deivce tree property name

Instead of inventing a new property name, let's use "wakeup-source" to
be consistent with other driver and subsystem bindings.

Signed-off-by: Shawn Guo <shawnguo@kernel.org>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>