ARM: OMAP3: fix build on !CONFIG_IOMMU_API

omap3isp depends on CONFIG_IOMMU_API, so avoid registering its
device (and defining its configuration structs) on !CONFIG_IOMMU_API.

This is generally nice to have, but more importantly, it fixes:

arch/arm/plat-omap/include/plat/iommu.h: In function 'dev_to_omap_iommu':
arch/arm/plat-omap/include/plat/iommu.h:135: error: 'struct
dev_archdata' has no member named 'iommu'
arch/arm/mach-omap2/devices.c: In function 'omap3_init_camera':
arch/arm/mach-omap2/devices.c:222: error: 'struct dev_archdata' has no
member named 'iommu'
make[1]: *** [arch/arm/mach-omap2/devices.o] Error 1
make: *** [arch/arm/mach-omap2] Error 2

Which happens because while setting up the omap3isp device we try
to access the (now nonexistent) iommu member of dev_archdata.

Compile tested with omap2plus_defconfig on today's:

commit e343a895
Merge: 06792c4d 193a667f
Author: Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue Jan 10 18:04:27 2012 -0800

    Merge tag 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost



Reported-by: Govindraj Raja <govindraj.raja@ti.com>
Reported-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Joerg Roedel <Joerg.Roedel@amd.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>