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>

include/common.h

@@ -616,6 +616,7 @@ ulong	lcd_setmem (ulong);
 ulong	video_setmem (ulong);
 
 /* arch/$(ARCH)/lib/cache.c */
+void	enable_caches(void);
 void	flush_cache   (unsigned long, unsigned long);
 void	flush_dcache_all(void);
 void	flush_dcache_range(unsigned long start, unsigned long stop);