soc: xtensa: intel_adsp: cavs: fix assert on L3_MEM_BASE_ADDR

The assert on L3_MEM_BASE_ADDR is incorrect, we need must convert
to uncached before use.

Fixes: ffd2121c ("soc: xtensa: intel_adsp: cavs: fix
  power_down_cavs() signature")
Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>