ANDROID: KVM: arm64: s2mpu: Add driver initializer

Create 'struct pkvm_iommu_ops' for the S2MPU and a new driver ID to the
list of IOMMU drivers. Implement the 'init' callback, accepting donated
memory from the host to back SMPTs. If the donation is successful,
the SMPTs are assigned to 'host_mpt'.

Export 'pkvm_iommu_s2mpu_register' for a kernel module to call to
register an S2MPU device. First call to this function will also
run the global S2MPU driver initializer.

Bug: 190463801
Signed-off-by: David Brazdil <dbrazdil@google.com>
Change-Id: I3d1aaf8535114beae956993674a2b436414f07c4