kernel: mem_domain: Use u8_t for number of partitions in struct

During system initialization, the global static variable (to
mem_domain.c) is initialized with the number of maximum partitions per
domain.  This variable is of u8_t type.

Assertions throughout the code will check ranges and test for overflow
by relying on implicit type conversion.

Use an u8_t instead of u32_t to avoid doubts.  Also, reorder the
k_mem_partition struct to remove the alignment hole created by reducing
sizeof(num_partitions).

Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>