iommu: rockchip: Free domain on .domain_free

IOMMU domain resource life is well-defined, managed
by .domain_alloc and .domain_free.

Therefore, domain-specific resources shouldn't be tied to
the device life, but instead to its domain.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>