units: Add BYTES_PER_*BIT

There is going to be a new user of the BYTES_PER_[K/M/G]BIT definition
besides possibly existing ones. Add them to the header.

Signed-off-by: Damian Muszynski <damian.muszynski@intel.com>
Reviewed-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Reviewed-by: Tero Kristo <tero.kristo@linux.intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>