drm/rockchip: DRM_ROCKCHIP should depend on ARCH_ROCKCHIP

drm/rockchip: DRM_ROCKCHIP should depend on ARCH_ROCKCHIP

Rockchip display hardware is only available on Rockchip SoCs.  Hence add
a dependency on ARCH_ROCKCHIP, to prevent asking the user about this
driver when configuring a kernel without Rockchip platform support.

Before, this dependency was implicit through a hard dependency on
ROCKCHIP_IOMMU.

Fixes: 0244539f9a4f3b56 ("drm/rockchip: Drop ROCKCHIP_IOMMU depend for DRM_ROCKCHIP")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Chaoyi Chen <chaoyi.chen@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patch.msgid.link/5db192d31cc51f027f107c01c01a353a0569ebf4.1768310045.git.geert+renesas@glider.be