iommu/arm-smmu: Make force_stage module param read-only in sysfs

Changing force_stage dynamically isn't supported by the driver and it
also doesn't make a whole lot of sense to change it once the SMMU is up
and running.

This patch makes the sysfs entry for the parameter read-only.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>