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/board.c b/arch/arm/lib/board.c
index c899839..a7fb251 100644
--- a/arch/arm/lib/board.c
+++ b/arch/arm/lib/board.c
@@ -452,11 +452,9 @@
 	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 */