powerpc/t104xrdb: Set DDR ODT to 75ohm

DDR-ODT require cfg_dram_type switch set properly as per DDR type.
T1040RDB, T1042RDB boards have DDR3L type DDR, so cfg_dram_type
should be set to OFF for DDR3L
Update t104xrdb/README for switch setting

Signed-off-by: Priyanka Jain <Priyanka.Jain@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>