Revert "soc: cavs_v25: increase core count default to 4"

This reverts commit af79664d.

The patch caused regression on TGL-H systems where only 2 DSP
cores are available. The Kconfig override used by existing SOF
application does not work with Zephyr. A quick fix using
Zephyr board revisions was considered, but review feedback
was that core count difference should be reflected in SoC-level
device tree. This will take more time to develop, so the revert
is needed to fix the immediate regression.

Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>