Kernel: Add support for various RK3399 based devices

I think it would be nice if you could boot vanilla Alpine on the large amount RK3399 based boards. The majority of these boards already have mainline support and can be booted from a mainline kernel with the default arm64 config. However they do not seem to work with Alpine's kernel config, although the DTBs are there.

I messed around for a bit on my Rock Pi 4, and one of the issues on it seems to be that the RK808 related flags are not enabled. After enabling these, the kernel finally booted but the uSD slot did not work, nor did the HDMI video output. I'm not sure why though. After testing on mainline's default config, everything works as expected.

This would likely help vanilla Alpine run on:

Radxa Rock Pi 4 (A/B/C)
Orange Pi 4
Pine64 RockPRO64
Pine64 PinePhone Pro
Pine64 PineBook Pro
Firefly-RK3399
ASUS Tinker Board 2
NanoPC T4
Others that I can't think of

Most of these boards from what I can gather have fairly similar hardware, so in theory supporting even one of these would likely accidentally make others work as well.

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