mt76: mt7615: introduce mt7615_register_map

In order to reuse mt7615 code adding support for mt7663e driver,
introduce mt7615e_reg_map since mt7663e and mt7615 rely on a
different base registers definitions.

Co-developed-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
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>