iicbus: Move rockchip driver into the common directory

No need to keep it under sys/arm64/rockchip
It's easier to find which controller we support by looking under one directory.
While here remove the condition on SOC option, device rk_i2c is enough as all
Rockchip SoC that we support have this controller.

Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D43184