ARM: OMAP: fix iommu, not mailbox

For some weird (freudian?) reason, commit 435792d "ARM: OMAP: make
iommu subsys_initcall to fix builtin omap3isp" unintentionally changed
the mailbox's initcall instead of the iommu's.

Fix that.

Reported-by: Fernando Guzman Lugo <fernando.lugo@ti.com>
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Joerg Roedel <Joerg.Roedel@amd.com>
Cc: Tony Lindgren <tony@atomide.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>