mt76: mt7615: move core shared code in mt7615-common module

Create mt7615-common module in order to collect shared code between usb
and mmio code. Move the following source files in mt7615-common module:
- main.c
- init.c
- mcu.c
- mac.c
- debugfs.c
- eeprom.c
- trace.c

Create the following source files for mmio only source code and move them
in mt7615e module:
- pci_init.c
- dma.c
- pci_mac.c

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>