iommu/exynos: Suppress unbinding to prevent system failure

Removal of IOMMU driver cannot be done reliably, so Exynos IOMMU driver
doesn't support this operation. It is essential for system operation, so
it makes sense to prevent unbinding by disabling bind/unbind sysfs
feature for SYSMMU controller driver to avoid kernel ops or trashing
memory caused by such operation.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
CC: stable@vger.kernel.org  # v4.2+
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>