drm/amdkfd: add svm ioctl API

Add svm (shared virtual memory) ioctl data structure and API definition.

The svm ioctl API is designed to be extensible in the future. All
operations are provided by a single IOCTL to preserve ioctl number
space. The arguments structure ends with a variable size array of
attributes that can be used to set or get one or multiple attributes.

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Signed-off-by: Alex Sierra <alex.sierra@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>