iommu/exynos: Fix arm64 allmodconfig build

The Exynos IOMMU driver uses the ARM specific dmac_flush_range() and
outer_flush_range() functions. This breaks the build on arm64 allmodconfig
in -next since support has been merged for some Exynos ARMv8 SoCs. Add a
dependency on ARM to keep things building until either the driver has the
ARM dependencies removed or the ARMv8 architecture code implements these
ARM specific APIs.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>