mtd: nand: omap2: Support parsing dma channel information from DT

Switch from dma_request_channel to allow passing dma channel
information from DT rather than hardcoding a value.

Also provide a handle to the GPMC's dev so it can be used to parse the DMA
channel information within the GPMC's DT node.

Performance Numbers via mtd_speedtest now that EDMA based prefetch works:

AM335x Performance numbers:
DMA
  CPULOAD Write: 54%  Read: 35%
  page write speed	-23% (vs non dma)
  page read speed	-35% (vs non dma)

NO DMA (prefetch-polled)
  CPULOAD Write: 98%  Read: 98%

AM437x Performance numbers:
DMA
  CPU LOAD Write: 56% Read: 36%
  page write speed	-16% (vs non dma)
  page read speed	-22% (vs non dma)

NO DMA (prefetch-polled)
  CPULOAD Write: 93%  Read: 93%

Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>