dt-bindings: net: realtek,rtl82xx: document wakeup-source property

dt-bindings: net: realtek,rtl82xx: document wakeup-source property

The RTL8211F PHY has two modes for a single INTB/PMEB pin:

1. INTB mode, where it signals interrupts to the CPU, which can
   include wake-on-LAN events.
2. PMEB mode, where it only signals a wake-on-LAN event, which
   may either be a latched logic low until software manually
   clears the WoL state, or pulsed mode.

In the case of (1), there is no way to know whether the interrupt to
which the PHY is connected is capable of waking the system. In the
case of (2), there would be no interrupt property in the PHY's DT
description, and thus there is nothing to describe whether the pin is
even wired to anything such as a power management controller.

There is a "wakeup-source" property which can be applied to any device
- see Documentation/devicetree/bindings/power/wakeup-source.txt

Case 1 above matches example 2 in this document, and case 2 above
matches example 3. Therefore, it seems reasonable to make use of this
existing specification, albiet it hasn't been converted to YAML.

Document the wakeup-source property in the device description, which
will indicate that the PHY has been wired up in such a way that it
can wake the system from a low power state.

We will use this in a rewrite of the existing broken Wake-on-Lan code
that was merged during the 6.16 merge window to support case 1. Case 2
can be added to the driver later without needing to further alter the
DT description. To be clear, the existing Wake-on-Lan code that was
recently merged has multiple functional issues.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/E1um9Xj-008kBx-72@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>