ARM: OMAP: dma: Make use of cpu_class_is_omap2() to avoid future patching.

cpu_class_is_omap2() contains all OMAP2+ devices. So update the DMA code
cpu checks accordingly so that there is no need to patch
the file for any future OMAP2+ devices.

In long run, all these attributes should come from hwmod dev_attr based
on DMA IP version.

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>