drm/amdgpu: Add initial kernel documentation for the amd_ip_block_type structure. v3

Added IP block section to amdgpu.rst.
Added more documentation to amd_ip_funcs.
Created documentation for amd_ip_block_type.

v2: Provides a more detailed DOC section on IP blocks
v3: Clarifies the IP block list. Adds info on IP block enumeration.

Signed-off-by: Ryan Taylor <ryan.taylor@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>