CBMEM: Define cbmem_top() just once for x86

It is expected this will always be a casted get_top_of_ram() call
on x86, no reason to do that under chipset.

Change-Id: I3a49abe13ca44bf4ca1e26d1b3baf954bc5a29b7
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/3972
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@google.com>

src/arch/x86/boot/cbmem.c

@@ -55,4 +55,12 @@ unsigned long __attribute__((weak)) get_top_of_ram(void)
 }
 #endif /* !__PRE_RAM__ */
 
-#endif
+#else
+
+void *cbmem_top(void)
+{
+	/* Top of cbmem is at lowest usable DRAM address below 4GiB. */
+	return (void *)get_top_of_ram();
+}
+
+#endif /* DYNAMIC_CBMEM */