ARM: OMAP: Fix dmaengine init for multiplatform

Otherwise omap dmaengine will initialized when booted
on other SoCs. Fix this by initializing the platform
device in arch/arm/*omap*/dma.c instead.

Cc: Russell King <linux@arm.linux.org.uk>
Cc: Dan Williams <djbw@fb.com>
Cc: Vinod Koul <vinod.koul@intel.com>
Tested-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>