This allows access to pages allocated through the driver with optional
IOMMU mapping.

v2: Fix number of bytes copied and add write method
v3: drop check for kmap return
Original-by: NChristian König <christian.koenig@amd.com>
Signed-off-by: NTom St Denis <tom.stdenis@amd.com>
Reviewed-by: NAlex Deucher <alexander.deucher@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>