commands: add spd_decode command ...

spd_decode command is used for decoding and printing
in human-readable format the information found
in memory module SPD EEPROMs.

There is the decode-dimms program from i2c-tools linux
package (see http://www.lm-sensors.org/wiki/I2CTools)
just for the same purpose.

Though spd_decode source code is based on decode-dimms
perl code there is a difference.

Signed-off-by: Alexander Smirnov <alllecs@yandex.ru>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>