Wakeup property of device is not Linux-specific, it describes intended
system behavior regardless of the OS being used. Therefore let's drop
"linux," prefix, and, while at it, use the same name as I2C bus does:
"wakeup-source".

We keep parsing old name to keep compatibility with old DTSes.

Cc: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>