lib: merge CRC16-CCITT into u-boot/crc.h

This merges the CRC16-CCITT headers into u-boot/crc.h to prepare for
rolling CRC16 into the hash infrastructure.  Given that CRC8, CRC32
and CRC32-C already have their prototypes in a single header file, it
seems a good idea to also include CRC16-CCITT in the same.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>