wifi: mt76: mt7921: fix suspend issue on MediaTek COB platform

MediaTek's controller driver on COB platform (e.g. MT8188) is
capable of controlling power supplies and reset pin of a component
(e.g. a WIFI chip) in power-on and power-off process.

Due to this optional feature, mt76 need to inform controller
that mt76 need to keep power during suspend. Otherwise WIFI will be
powered off when system enters suspend process.

The "wakeup-source" property was used for the device that need
this to go into suspend mode so that mt76 suspend handler doesn't
fail and the system is able to enter into a suspend state.

An example:
wifi: mt7921@0 {
	wifi0{
		reg = <0x0000 0 0 0 0>;
		wakeup-source;
	};
};

Signed-off-by: Michael Lo <michael.lo@mediatek.com>
Signed-off-by: Ming Yen Hsieh <MingYen.Hsieh@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>