iommu/exynos: Replace non-existing big-endian Kconfig option

Wrong Kconfig option was used when adding warning for untested
big-endian capabilities. There is no CONFIG_BIG_ENDIAN option.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>