ARM: OMAP2+: Remove unused legacy code for DMA

We are now booting all mach-omap2 in device tree only mode.
Any code that is only called in legacy boot mode where
of_have_populated_dt() is not set is safe to remove now.

Let's leave the dummy omap2_system_dma_init_dev() check
in place for now to avoid a pointless merge conflict with
tusb6010 dmaengine conversion as pointed out by Peter
Ujfalusi <peter.ujfalusi@ti.com>.

Signed-off-by: Tony Lindgren <tony@atomide.com>