soc: intel_adsp/ace: update SOC_TOOLCHAIN_NAME

Update the SOC_TOOLCHAIN_NAME to intel_ace15_mtpm so that
we use the correct overlay in Xtensa HAL module. Note that
ace20_lnl will also be using this as well. That will change
once we have a proper toolchain for ace20_lnl.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>