powerpc/dma/raidengine: add raidengine device

The RaidEngine is a new Freescale hardware that used for parity
computation offloading in RAID5/6.

This patch adds the device node in device tree and related binding
documentation.

Signed-off-by: Harninder Rai <harninder.rai@freescale.com>
Signed-off-by: Naveen Burmi <naveenburmi@freescale.com>
Signed-off-by: Xuelin Shi <b29237@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>