mt76: mt7915: add debugfs knobs for MCU utilization

Add debugfs knobs to read MCU utilization, which helps user know
firmware status more easily to narrow down CPU bottleneck issues.

Co-developed-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>