ANDROID: KVM: arm64: s2mpu: Create SysMMU_SYNC driver

SysMMU_SYNC devices expose an interface to start a sync counter and
poll its SFR until the device signals that all memory transactions in
flight at the start have drained. This gives the hypervisor a reliable
indicator that S2MPU invalidation has fully completed and all new
transactions will use the new MPTs.

Add a new pKVM IOMMU driver that the host can use to register
SysMMU_SYNCs. Each device is expected to be a supplier to exactly one
S2MPU (parent), but multiple SYNCs can supply a single S2MPU.

To keep things simple, the SYNCs do not implement suspend/resume and are
assumed to follow the power transitions of their parent.

Following an invalidation, the S2MPU driver iterates over its children
and waits for each SYNC to signal that its transactions have drained.
The algorithm currently waits on each SYNC in turn. If latency proves to
be an issue, this could be optimized to initiate a SYNC on all powered
devices before starting to poll.

Bug: 190463801
Signed-off-by: David Brazdil <dbrazdil@google.com>
Change-Id: I45b832fd11d76b65987935c8548e2a214ee2fa2a