mediatek: filogic: add Adtran SmartRG Mount Stuart series

mediatek: filogic: add Adtran SmartRG Mount Stuart series

Common specifications:
 * Mediatek MT7988A (4x Cortex-A73, up to 1.8 GHz clock speed)
 * 8 GiB eMMC
 * 2 GiB DDR4 RAM
 * 1x 10000M/1000M/100M + 3x 1000M/100M/10M LAN ports
 * MT7996 Tri-band (2.4G, 5G, 6G) 4T4R 802.11be Wi-Fi
 * Airoha AG3352 GPS
 * Renesas DA14531MOD Bluetooth
 * 2 buttons (Reset, Mesh/WPS)
 * uC-controlled RGB LED via I2C
 * 2x LED for each 1G port, 3x LED for each 10G port
 * USB 3.0 type A port
 * 3.3V-level 115200 baud UART console via 4-pin Dupont connector
   exposed at the bottom of the device
 * USB-C PD power input

SDG-8733: 1x 10000M/1000M/100M WAN port
SDG-8734: 1x USXGMII/10GBase-R/5GBase-R/2500Base-X/1000Base-X/SGMII SFP+

Both models are also available in versions including 2x FXS POTS interfaces
for analog phones. Those interfaces are not supported by OpenWrt.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>