arm64: Only allow dcache disabled in SPL builds

Now that we have an easy way to describe memory regions and enable the MMU,
there really shouldn't be anything holding people back from running with
caches enabled on AArch64. To make sure people catch early if they're missing
on the caching fun, give them a compile error.

Signed-off-by: Alexander Graf <agraf@suse.de>

arch/arm/cpu/armv8/cache_v8.c

@@ -566,6 +566,15 @@ void mmu_set_region_dcache_behaviour(phys_addr_t start, size_t size,
 
 #else	/* CONFIG_SYS_DCACHE_OFF */
 
+/*
+ * For SPL builds, we may want to not have dcache enabled. Any real U-Boot
+ * running however really wants to have dcache and the MMU active. Check that
+ * everything is sane and give the developer a hint if it isn't.
+ */
+#ifndef CONFIG_SPL_BUILD
+#error Please describe your MMU layout in CONFIG_SYS_MEM_MAP and enable dcache.
+#endif
+
 void invalidate_dcache_all(void)
 {
 }