drm/rockchip: Drop ROCKCHIP_IOMMU depend for DRM_ROCKCHIP

drm/rockchip: Drop ROCKCHIP_IOMMU depend for DRM_ROCKCHIP

On the RK3506 platform, there is no iommu hardware. And even on
platform that have iommu hardware, it should be possible to use
VOP without enabling iommu. In this case, a contiguous memory
space like CMA should be used.

So this patch removes the dependency on ROCKCHIP_IOMMU
with an 'optional-dependency'.

Signed-off-by: Chaoyi Chen <chaoyi.chen@rock-chips.com>
[added reference to optional-dependies description]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patch.msgid.link/20251106020632.92-9-kernel@airkyi.com