kernel: mem_domain: Fix compile issues

Commit 08de658e ("kernel: mem_domain: Check for overlapping regions
when considering W^X") introduced some compile issues on various
platforms.

The k_mem_partition_attr_t member is attr not attrs.  Also, fix an issue
where sane_partition_domain neesd a pointer to a parition.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>