Allow preemption timeout to be specified as a config option.

v2: Change unit to microseconds (Tejas)
v3: Remove get_default_preempt_timeout()
Reviewed-by: Tejas Upadhyay <tejas.upadhyay@intel.com>
Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>