powerpc/pseries/svm: Make mem_encrypt.h self contained

Add the missing forward declarations and includes so it does not have
implicit dependencies. mem_encrypt.h is a public header imported by
drivers. Users should not have to guess what include files are needed.

Resolves a kbuild splat:

   In file included from drivers/iommu/generic_pt/fmt/iommu_amdv1.c:15:
   In file included from drivers/iommu/generic_pt/fmt/iommu_template.h:36:
   In file included from drivers/iommu/generic_pt/fmt/amdv1.h:23:
   In file included from include/linux/mem_encrypt.h:17:
>> arch/powerpc/include/asm/mem_encrypt.h:13:49: warning: declaration of 'struct device' will not be visible outside of this function [-Wvisibility]
      13 | static inline bool force_dma_unencrypted(struct device *dev)

Fixes: 879ced2b ("iommupt: Add the AMD IOMMU v1 page table format")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202511161358.rS5pSb3U-lkp@intel.com/


Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Vasant Hegde <vasant.hegde@amd.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>