soc/intel_adsp: fix icache initialization

XCHAL_HAVE_ICACHE_DYN_ENABLE is not set for any Intel cAVS
hardware, so MEMCTL configuration is not done properly leaving
icache disabled. This can be seen as ~10X slowness when running
code on non-primary cores. Fix the issue by using XCHAL_USE_MEMCTL
to check for MEMCTL usage.

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