kernel: mem_domain: k_mem_partition is now placed in kernel memory.

The k_mem_partition structs need to be placed in the kernel memory.
This patch ensures that these structs are placed correctly.
Also when a struct k_mem_domain is declared it is advised to add
__kernel.

Signed-off-by: Adithya Baglody <adithya.nagaraj.baglody@intel.com>