ARM: OMAP: DMA: Remove __REG access

Remove __REG access in DMA code, use dma_read/write instead:

- dynamically set the omap_dma_base based on the omap type
- omap_read/write becomes dma_read/write
- dma channel registers are read with dma_ch_read/write

Cc: David Brownell <david-b@pacbell.net>
Cc: linux-usb@vger.kernel.org
Signed-off-by: Tony Lindgren <tony@atomide.com>