ISP hw block is supported in some of the AMD GPU versions, add support
to discover ISP IP in amdgpu_discovery.

v2: squash in documentation update (Alex)
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Pratap Nirujogi <pratap.nirujogi@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>