mt76: mt7915: add debugfs knob for RF registers read/write

Add RF registers read/write support for debugging RF issues, which
should be processed by mcu commands.
The index of rf registers use the generic regidx, and are combined
with two parts: WF selection [31:28] and offset [27:0].

Reviewed-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>