AMD/IOMMU: use bit field for control register

AMD/IOMMU: use bit field for control register

Also introduce a field in struct amd_iommu caching the most recently
written control register. All writes should now happen exclusively from
that cached value, such that it is guaranteed to be up to date.

Take the opportunity and add further fields. Also convert a few boolean
function parameters to bool, such that use of !! can be avoided.

Because of there now being definitions beyond bit 31, writel() also gets
replaced by writeq() when updating hardware.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Brian Woods <brian.woods@amd.com>