ARM (ARM926ejs): add data cache support, tested on magnesium and tx25 board

Enable "cache" command on tx25 and magnesium board and test performance.

    Test 1: Loading 127 MB of data from NAND flash into RAM:

    Instr. Cache        off     on      on
      Data Cache        off     off     on
    --------------------------------------------------
    magnesium           32,6s   22,5s   30s     = x 1,09
    tx25 (29MB only)    9,69s   5,05s   8,16s   = x 1,19

    Test 2: uncompressing a gzipped image from RAM to RAM
            (size compressed: 6.5 MiB, uncompressed: 35 MiB):

    Instr. Cache        off     on      on
      Data Cache        off     off     on
    --------------------------------------------------
    magnesium           4,25s   2,08s   1,72s   = x 2,47
    tx25                4,82s   2,04s   1,84s   = x 2,62

Portions of this work were supported by funding from
the CE Linux Forum.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Alessandro Rubini <rubini@gnudd.com>