omap: sdma: Limit the secure reserve channel fix for omap3

The commit 'ba50ea7e' reserves DMA channels 0 and 1 on high
security devices, in order to avoid collision between kernel
dma transfers and ROM code dma transfers.

This fix is applicable only for OMAP3 so add an appropriate
check.

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
CC: Kalle Jokiniemi <kalle.jokiniemi@digia.com>
CC: Kevin Hilman <khilman@deeprootsystems.com>
Reviewed-by: Kevin Hilman <khilman@deeprootsystems.com>
Reviewed-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>