iommu: Tidy up window attributes

The external interface to get/set window attributes is already
abstracted behind iommu_domain_{get,set}_attr(), so there's no real
reason for the internal interface to be different. Since we only have
one window-based driver anyway, clean up the core code by just moving
the DOMAIN_ATTR_WINDOWS handling directly into the PAMU driver.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>