iommu/ipmmu-vmsa: Remove platform data handling

The IPMMU driver is using DT these days, and platform data is no longer
used by the driver. Remove unused code.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Joerg Roedel <jroedel@suse.de>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Joerg Roedel <jroedel@suse.de>