How should a wakeup source for a device be configured in code.
I could not find it documented anywhere.

Check out drivers/input/keyboard/samsung-keypad.c. It is configured as
a wakeup source:-
1. device_init_wakeup() - configuring the device to be of wakeup type
2. enable_irq_wake() - configuring the interrupt to be of wakeup type.

True, I could not find the documentation for the same.

Regards,
Ayan Kumar Halder