mediatek: add Linksys E8450 support

mediatek: add Linksys E8450 support

The Linksys E8450, also known as Belkin RT3200, is a dual-band
IEEE 802.11bgn/ac/ax router based on MediaTek MT7622BV and
MediaTek MT7915AN chips.

FCC: K7S-03571 and K7S-03572

Hardware highlights:
 - CPU: MediaTek MT7622BV (2x ARM Cortex-A53 @ 1350 MHz max.)
 - RAM: 512MB DDR3
 - Flash: 128MB SPI-NAND (2k+64)
 - Ethernet: MT7531BE switch with 5 1000Base-T ports
             CPU port connected with 2500Base-X
 - WiFi 2.4 GHz: 802.11bgn 4T4R built-in antennas
                 MT7622VB built-in
 - WiFi   5 GHz: 802.11ac/ax 4T4R built-in antennas
                 MT7915AN chip on-board via PCIe
                 MT7975AN front-end
 - Buttons: Reset and WPS
 - LEDS: 3 user controllable LEDs, 4 wired to switch
 - USB: USB2.0, single port
 - no Bluetooth (supported by SoC, not wired on board)
 - Serial: JST PH 2.0MM 6 Pin connector inside device
            ----_____________----
           [  GND RX - TX  -  -  ]
            ---------------------
 - JTAG:   unpopulated ARM JTAG 20-pin connector (works)

This commit adds support for the device in a way that is compatible
with the vendor firmware's bootloader and dual-boot flash layout, the
resulting image can directly be flashed using the vendor firmware.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>