powerpc/eeprom: update MAX_NUM_PORTS to adapt non-256-bytes EEPROM

Some boards use System EEPROM with 128-bytes instead of 256-bytes.
Since we regard 256-bytes EEPROM as standard EEPROM with default
value for MAX_NUM_PORTS. For those non-256-bytes EEPROM, we can
redefine MAX_NUM_PORTS in board-specific file to override the
default MAX_NUM_PORTS.

This patch doesn't impact on previous existing boards.

Signed-off-by: Shengzhou Liu <Shengzhou.Liu@freescale.com>