staging/rdma/hfi1: Add comment for spinlock_t definition

Add comments describing the spinlock for spinlock_t definitions to
fix checkpatch check:
CHECK: spinlock_t definition without comment

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Jubin John <jubin.john@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>