soc: rockchip: Restrict to ARCH_ROCKCHIP

By definition this directory contains drivers that are specific to the
Rockchip architecture. All Kconfig options should therefore depend on
ARCH_ROCKCHIP to avoid exposing these symbols on other architectures.
For example, this options currently shows up as new when doing an
incremental build on PowerPC.

Signed-off-by: Thierry Reding <treding@nvidia.com>

[add COMPILE_TEST alternative condition]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>