xtensa: cavs_v18: use uncached addresses to support SMP

SMP support on cAVS is implemented by using uncached addresses
for all writable data sections except for stack, i.e. for .data,
.bss and some other specialised ones. So far that has been
implemented for cAVS 1.5. This patch does the same for cAVS 1.8.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>