soc: boards: xtensa: intel_adsp_ace: Rename with HWMv2

Rename for better HWMv2 use:

  SoC - `intel_ace15_mtpm` --> `ace15_mtpm`
  SoC - `intel_ace20_lnl`  --> `ace20_lnl`

Resulting shortened name for boards:

  `intel_adsp_ace15_mtpm/intel_ace15_mtpm` -->
  `intel_adsp_ace15_mtpm/ace15_mtpm`

  `intel_adsp_ace20_lnl/intel_ace20_lnl` -->
  `intel_adsp_ace20_lnl/ace20_lnl`

Signed-off-by: Dmitrii Golovanov <dmitrii.golovanov@intel.com>