iommu/exynos: Add support for runtime_pm

This patch fixes support for runtime power management for SYSMMU
controllers, so they are enabled when master device is attached.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Joerg Roedel <jroedel@suse.de>