This patch clear and disable the data cache for vxWorks.

The entry point sysInit(int) intended by Windriver to be called from
the vxWorks bootrom, a very small vxWorks system.
The routine is called by the go() handler in the bootrom, that clears
the cache from start of image to end of usable memory.

The PowerPC implementations only invalidates and disable the cache,
the ARM implementations also flush it.

U-Boot will be on the safe side, if it disables the data cache before
calling vxWorks sysInit(int).

Signed-off-by: Reinhard Arlt <reinhard.arlt@esd.eu>