Add data cache support for arm1136 systems.

Enable "cache" command on Qong 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
    --------------------------------------------------
    QONG (ARM11)	177s	95s	43s	= x 4.1

    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
    --------------------------------------------------
    QONG (ARM11)	1.54s	0.95s	0.18s	= x 8.6

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

Signed-off-by: Heiko Schocher <hs@denx.de>