iommu/exynos: prevent building on big-endian kernels

Since we print the correct warning, an allmodconfig build is no longer
clean but always prints it, which defeats compile-testing:

drivers/iommu/exynos-iommu.c:58:2: error: #warning "revisit driver if we can enable big-endian ptes" [-Werror=cpp]

This replaces the #warning with a dependency, moving warning text into
a comment.

Fixes: 1f59adb1 ("iommu/exynos: Replace non-existing big-endian Kconfig option")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>