tests: Enable the counter_basic_api test on mec172x board

1. enable timer5 on dts
2. Replace all BTMR_Type with struct btmr_regs, to make it compatiable
   on both mec 15 and 17
3. As MEC15 and MEC17 have different ways to access the register, use
   "#if defined" to access counter_cfg->girq_id on different board.

Signed-off-by: Hu Zhenyu <zhenyu.hu@intel.com>