soc/xtensa/nxp_adsp/CMakeLists.txt: use new WEST_SIGN_OPTS variable

Align `soc/nxp_adsp` with new `WEST_SIGN_OPTS` option added to
`soc/intel_adsp` by recent commit d98a7c2f ("soc: xtensa: cmake: add
new WEST_SIGN_OPTS variable").

This allows per-board rimage customization at the CMake level. Example
in `zephyr/boards/xtensa/nxp_adsp_NEWBOARD/board.cmake`:

  set(WEST_SIGN_OPTS -- -c rimage/config/special.toml)

Signed-off-by: Marc Herbert <marc.herbert@intel.com>