[arm64] drivers/clk/mediatek: Set MT7986 clock driver builtin

Mediatek MT7986 SoC used by BananaPi R3/R3-Mini routers enabled by b75568e7 ("[arm64] Add MediaTek MT7986a SoC support") and 5f005754 ("[arm64] Add Bananapi BPI-R3 support").

However, for MT7986 clock driver is required before userspace. If built as a module, d-i will not load because initramfs cannot reached. Instead of building it as a module, let's build it as a builtin. This will start d-i successfully.

Closes: 1094195 Link: https://bugs.debian.org/1094195