mtd: cfi_mtd: Use DMA for reads

When possible use DMA for reading from CFI flash, this provides upto 5x
improvement in read performance with high speed CFI compliant flashes
like HyperFlash.

Code will gracefully fallback to CPU copy when DMA is unavailable.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Stefan Roese <sr@denx.de>