iommu clocks are optional, so the driver should not fail if they are not
present. Instead just set the number of clocks to 0, which the clk-blk APIs
can handle just fine.

Fixes: f2e3a5f5 ("iommu/rockchip: Control clocks needed to access the IOMMU")
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Tested-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>