arm64: Add MediaTek SoC support skeleton

[hayzam_gmail.com]

Summary

Add the SOC_MEDIATEK and SOC_MEDIATEK_MT7986 options and std.mediatek,
include it in GENERIC, and add a DTB module that builds the Banana Pi
BPI-R3 (MT7986A) DTB.

Sponsored by: Alchemilla Ventures Private Limited

Test Plan

Builds on arm64 GENERIC. Tested together with the UART driver, see
D61191.