mt76: mt7915: add mt7915_mmio_probe() as a common probing function

Add mt7915_mmio_probe() which will be used for the upcoming devices.
This is an intermediate patch to add mt7916 support.

Co-developed-by: Sujuan Chen <sujuan.chen@mediatek.com>
Signed-off-by: Sujuan Chen <sujuan.chen@mediatek.com>
Co-developed-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Bo Jiao <Bo.Jiao@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>