iommu: make inclusion of riscv directory conditional

Nothing in there is active if CONFIG_RISCV_IOMMU is not enabled, so the whole
directory can depend on that switch as well.

Fixes: 5c0ebbd3 ("iommu/riscv: Add RISC-V IOMMU platform device driver")
Signed-off-by: Rolf Eike Beer <eb@emlix.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/2235451.Icojqenx9y@devpool92.emlix.com


Signed-off-by: Joerg Roedel <jroedel@suse.de>