mt76: mt7615: add radar pattern test knob to debugfs

Introduce mt7615_mcu_rdd_send_pattern routine to trigger a radar pattern
detection. Moreover move debugfs related routines in a dedicated source
file.

Suggested-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>