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: 1f59adb176 ("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>

drivers/iommu/Kconfig

@@ -253,6 +253,7 @@ config TEGRA_IOMMU_SMMU
 config EXYNOS_IOMMU
 	bool "Exynos IOMMU Support"
 	depends on ARCH_EXYNOS && MMU
+	depends on !CPU_BIG_ENDIAN # revisit driver if we can enable big-endian ptes
 	select IOMMU_API
 	select ARM_DMA_USE_IOMMU
 	help

drivers/iommu/exynos-iommu.c

@@ -54,10 +54,6 @@ typedef u32 sysmmu_pte_t;
 #define lv2ent_small(pent) ((*(pent) & 2) == 2)
 #define lv2ent_large(pent) ((*(pent) & 3) == 1)
 
-#ifdef CONFIG_CPU_BIG_ENDIAN
-#warning "revisit driver if we can enable big-endian ptes"
-#endif
-
 /*
  * v1.x - v3.x SYSMMU supports 32bit physical and 32bit virtual address spaces
  * v5.0 introduced support for 36bit physical address space by shifting