iommu: rockchip: fix building without CONFIG_OF

We get a build error when compiling the iommu driver without CONFIG_OF:

drivers/iommu/rockchip-iommu.c: In function 'rk_iommu_of_xlate':
drivers/iommu/rockchip-iommu.c:1101:2: error: implicit declaration of function 'of_dev_put'; did you mean 'of_node_put'? [-Werror=implicit-function-declaration]

This replaces the of_dev_put() with the equivalent
platform_device_put().

Fixes: 5fd577c3

 ("iommu/rockchip: Use OF_IOMMU to attach devices automatically")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>