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>

board/freescale/common/sys_eeprom.c

@@ -18,7 +18,11 @@
 #endif
 
 #ifdef CONFIG_SYS_I2C_EEPROM_NXID
+/* some boards with non-256-bytes EEPROM have special define */
+/* for MAX_NUM_PORTS in board-specific file */
+#ifndef MAX_NUM_PORTS
 #define MAX_NUM_PORTS	23
+#endif
 #define NXID_VERSION	1
 #endif