mediatek: add Adtran SmartRG SDG-8733A
Specification is similar to other devices of the MT Stuart series:
* Mediatek MT7988D (3x Cortex-A73, up to 1.8 GHz clock speed)
* 8 GiB eMMC
* 2 GiB DDR4 RAM
* 2500M/1000M/100M LAN port
* 10000M/5000M/2500M/1000M/100M/10M WAN port
* MT7992 Tri-band (2.4G, 5G, 6G) 2T2R+3T3R+3T3R 802.11be Wi-Fi
* Renesas DA14531MOD Bluetooth
* 2 buttons (Reset, Mesh/WPS)
* uC-controlled RGB LED via I2C
* 2x LED for the 2.5G port, 3x LED for the 10G 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
Signed-off-by: Daniel Golle <daniel@makrotopia.org>