Introduce is_mt7925() helper for new chipset. mt7925 runs the same
firmware download and mmio map flow as mt7921.

This is a preliminary patch to support mt7925 driver.
Co-developed-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>