arm: call iommu_init before of_platform_populate

We need to ensure that the IOMMUs in the system have a chance to perform
some basic initialisation before we start adding masters to them.

This patch adds a call to of_iommu_init before of_platform_populate.

Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>