mt76: mt7915: add LED support

Initialize brightness_set and blink_set callbacks to enable LED support.
The base 0x7c000000 is MCU space so driver can't touch it, and should
map the LED base to 0x18000000 host's view.

Signed-off-by: MeiChia Chiu <meichia.chiu@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Money Wang <money.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>