arm: do not force d-cache enable on all boards

c2dd0d45540397704de9b13287417d21049d34c6 added dcache_enable()
to board_init_r(). This enables d-cache for all ARM boards.
As a result some of the arm boards that are not cache-ready
are broken. Revert this change and allow platform code to
take the decision on d-cache enabling.

Also add some documentation for cache usage in ARM.

Signed-off-by: Aneesh V <aneesh@ti.com>
diff --git a/arch/arm/lib/cache.c b/arch/arm/lib/cache.c
index 92b61a2..b545fb7 100644
--- a/arch/arm/lib/cache.c
+++ b/arch/arm/lib/cache.c
@@ -53,3 +53,15 @@
 }
 void	flush_dcache_all(void)
 	__attribute__((weak, alias("__flush_dcache_all")));
+
+
+/*
+ * Default implementation of enable_caches()
+ * Real implementation should be in platform code
+ */
+void __enable_caches(void)
+{
+	puts("WARNING: Caches not enabled\n");
+}
+void enable_caches(void)
+	__attribute__((weak, alias("__enable_caches")));