soc: xtensa: intel_adsp: cavs: fix power_down_cavs() signature

The second argument 'uint32_t *hpsram_pg_mask' must be a cached
pointer and this needs to be reflected in function prototype.

Fixes sparse warning:
/zep_workspace/zephyr/soc/xtensa/intel_adsp/cavs/power.c:106:63:
warning: incorrect type in argument 2 (different address spaces)

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