board: cssi: Refactor EEPROM read

board: cssi: Refactor EEPROM read

On cmpc885 board, the ethernet addresses are stored in an
EEPROM that is accessed through SPI.

A 3 bytes command is sent to the chip then the content
gets read. At the time being a single block access is
performed, ignoring the first 3 bytes read.

Reword the SPI transfer to first send 3 bytes then
receive the content of the EEPROM so that there don't be
3 dummy bytes at the beginning of the buffer.

And move the function into common.c so that it can be
reused by the board that will be added in a future patch.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>