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

c2dd0d4554 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>

arch/arm/lib/board.c

@@ -452,11 +452,9 @@ void board_init_r(gd_t *id, ulong dest_addr)
 	gd->flags |= GD_FLG_RELOC;	/* tell others: relocation done */
 
 	monitor_flash_len = _end_ofs;
-	/*
-	 * Enable D$:
-	 * I$, if needed, must be already enabled in start.S
-	 */
-	dcache_enable();
+
+	/* Enable caches */
+	enable_caches();
 
 	debug("monitor flash len: %08lX\n", monitor_flash_len);
 	board_init();	/* Setup chipselects */

arch/arm/lib/cache.c

@@ -53,3 +53,15 @@ void	__flush_dcache_all(void)
 }
 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")));