pistachio: set correct CBMEM top address

This is required for proper dynamic CBMEM operation.

BRANCH=none
BUG=chrome-os-partner:31438
TEST=urara ramstage does not crash anymore in CBMEM initialization

Change-Id: I5d44907443724891c8cff805208a7a3bb8e36752
Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/232292
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/soc/imgtec/pistachio/cbmem.c

@@ -21,10 +21,10 @@
 
 #include <cbmem.h>
 #include <stdlib.h>
+#include <symbols.h>
 
 void *cbmem_top(void)
 {
-	uintptr_t top = MIN(CONFIG_DRAM_SIZE_MB, 256) << 20;
-	return (void *)(top + CONFIG_SYS_SDRAM_BASE);
+	return _dram + (CONFIG_DRAM_SIZE_MB << 20);
 }