arm64: Add a new SOC_ROCKCHIP option

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.

Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D48286