85xx: socrates: fix DDR SDRAM tlb entry configuration

since commit be0bd823
tlb entry for socrates DDR SDRAM will be reconfigured
by setup_ddr_tlbs() from initdram() causing an
inconsistency with previously configured DDR SDRAM tlb
entry from tlb_table:

socrates>l2cam 7 9
IDX  PID      EPN  SIZE V TS           RPN U0-U3 WIMGE UUUSSS
  7 : 00 00000000 256MB V  0 -> 0_00000000  0000 -I-G- ---RWX
  8 : 00 00000000 256MB V  0 -> 0_00000000  0000 ----- ---RWX
  9 : 00 10000000 256MB V  0 -> 0_10000000  0000 ----- ---RWX

This patch makes the presence of the DDR SDRAM tlb entry in
the tlb_table dependent on CONFIG_SPD_EEPROM to avoid this
inconsistency.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Acked-by: Andy Fleming <afleming@freescale.com>