boards: hsdk: let mdb runner connect cores according to configuration

let mdd runner connect connect cores according to CONFIG_MP_NUM_CPUS,
e.g.
   * CONFIG_MP_NUM_CPUS = 2, just connect 2 cores
   * CONFIG_MP_NUM_CPUS = 1, just connect 1 core
   * CONFIG_MP_NUM_CPUS = 4, connect all 4 cores

Signed-off-by: Wayne Ren <wei.ren@synopsys.com>