wifi: mt76: mt7915: enable per-phy led support

Introduce the capability to support per-phy led blinking. This is needed
for devices supporting dbdc.

Tested-by: Frank Wunderlich <frank-w@public-files.de>
Co-developed-by: Ryder Lee <ryder.Lee@mediatek.com>
Signed-off-by: Ryder Lee <ryder.Lee@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>