clover: Stub implementation of CL 1.2 sub-devices.

clover: Stub implementation of CL 1.2 sub-devices.

The implementation is basically a NOP but it conforms with OpenCL 1.2.

[ Francisco Jerez: Initialize property return buffer for
  CL_DEVICE_PARTITION_PROPERTIES, CL_DEVICE_PARTITION_TYPE,
  CL_DEVICE_PARTITION_AFFINITY_DOMAIN, and make the latter a scalar
  rather than a vector.  Some clean-up and code style fixes. ]

Reviewed-by: Francisco Jerez <currojerez@riseup.net>