ARM: dts: exynos: Fix IOMMU support for GScaler devices on Exynos5250

The proper name for the property, which assign given device to IOMMU is
'iommus', not 'iommu'. Fix incorrect name and let all GScaler devices
to be properly handled when IOMMU support is enabled.

Reported-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Fixes: 6cbfdd73 ("ARM: dts: add sysmmu nodes for exynos5250")
Cc: <stable@vger.kernel.org> # v4.8+
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>