kernel: typedef k_mem_domain_t and _mem_domain_info_t

Create alias for struct k_mem_domain into k_mem_domain_t,
and struct _mem_domain_info into _mem_domain_info_t via
typedef. These are needed to generate offsets via
GEN_OFFSET_SYM.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>