byteorder: Add all remaining helpers for both little and big endian

- making bswap_16 and bswap_32 for both
- adding sys_be16_to_cpu, sys_cpu_to_be16, sys_be32_to_cpu and
  sys_cpu_to_be32

Change-Id: I3aacc91590ee584d83c512d78a9287380969d9aa
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>