Add data cache support for ARM V7 systems. Used cache flush
functions from linux:arch/arm/mm/cache-v7.S developed from
Catalin Marinas.

Enable "cache" command on Beagle 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
    --------------------------------------------------
    Beagle (Cortex A8)	116s	106s	30.3s	= x 3.8

    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
    --------------------------------------------------
    Beagle (Cortex A8)	1.84s	1.64s	0.12s	= x 15.3

Portions of this work were supported by funding from
the CE Linux Forum.
Signed-off-by: NHeiko Schocher <hs@denx.de>
Reviewed-by: Ben Gardiner<bengardiner@nanometrics.ca>