samples: change name for the mec15xxevb_assy6853 pm sample

Power management sample for that board has very
confusing name that should be changed to something
more understandable.
Current name sample.board.sample.board.mec15xxevb_assy6853
I suggest to have new name
sample.board.power_management.mec15xxevb_assy6853

Signed-off-by: Maksim Masalski <maksim.masalski@intel.com>