kernel: remove MEM_PARTITION_ENTRY macro

MEM_PARTITION_ENTRY is problematic, as it assumes that
struct k_mem_partition contains a k_mem_partition_attr_t
field, which is only true if Memory Protection is supported.
Additionally, it works with k_mem_partition_attr_t being a
single element object (scalar or single element structure).
This commit removes the macro function and updates macro
K_MEM_PARTITION_DEFINE() (MEM_PARTITION_ENTRY has only been
used in that macro function definition).

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>