The code calculates the DDR2 SPD checksum as per JEDEC standard
No 21-C Appendix X (revision 1.2)

The code is based on the equivalent files from U-Boot version
git-a71d45d.

Signed-off-by: Renaud Barbier <renaud.barbier@ge.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>