arm64: Add a new SOC_ROCKCHIP option

[manu]

Summary

A lot of drivers are shared between all rockchip SoCs, each time we add
suppot for a new SoC we need to add the options in the files.arm64 lines.
Add a new option SOC_ROCKCHIP that will help simplify this file.

[andrew]

Can you add a comment that SOC_ROCKCHIP_* depend on SOC_ROCKCHIP

[manu]

In D48286#1101250, @andrew wrote:

Can you add a comment that SOC_ROCKCHIP_* depend on SOC_ROCKCHIP

Sure, where do you think it would be best, in options.arm64 ?

[andrew]

Yes, that's probably the best place for it