dmaengine: correct invalid assumptions in the Kconfig text

This patch corrects recently changed (and now invalid) Kconfig descriptions
for the DMA engine framework:

 - Non-Intel(R) hardware also has DMA engines;
 - DMA is used for more than memcpy and RAID offloading.

In fact, on most platforms memcpy and RAID aren't factors, and DMA
exists so that peripherals can transfer data to/from memory while
the CPU does other work.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>