fsl_p2020rdbpca: add support for the ONIE sys_eeprom command

The FSL_P2020RDBPCA has no EEPROM, hence the common ONIE sys_eeprom
command was not be demonstrated.

This patch uses a NOR flash sector to store the board information
EEPROM data.  Also the sys_eeprom command.

The MTD partitions now look like:

FSL_P2020RDBPCA:/ # cat /proc/mtd
dev:    size   erasesize  name
mtd0: 00040000 00020000 "NOR (RO) Vitesse-7385 Firmware"
mtd1: 00020000 00020000 "board_eeprom"
mtd2: 00b00000 00020000 "open"
mtd3: 00400000 00020000 "onie"
mtd4: 00020000 00020000 "uboot-env"
mtd5: 00080000 00020000 "uboot"

where mtd1 holds the EEPROM data.

A reasonable way to program the EEPROM from U-Boot using the
sys_eeprom command is:

=> sys_eeprom set 0x21 P2020RDB-PCA
=> sys_eeprom set 0x23 fake-serial-0123456789
=> sys_eeprom set 0x24 00:04:9f:02:80:a4
=> sys_eeprom set 0x28 fsl_p2020rdbpca-r0
=> sys_eeprom set 0x2a 1
=> sys_eeprom set 0x2d Freescale
=> sys_eeprom
TlvInfo Header:
   Id String:    TlvInfo
   Version:      1
   Total Length: 87
TLV Name             Code Len Value
-------------------- ---- --- -----
Product Name         0x21  12 P2020RDB-PCA
Serial Number        0x23  22 fake-serial-0123456789
Base MAC Address     0x24   6 00:04:9F:02:80:A4
Platform Name        0x28  18 fsl_p2020rdbpca-r0
MAC Addresses        0x2A   2 1
Vendor Name          0x2D   9 Freescale
CRC-32               0xFE   4 0x5B0EA7B7
Checksum is valid.
=> sys_eeprom write
Un-Protected 1 sectors

. done
Erased 1 sectors
9.Protected 1 sectors
Programming passed.