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/mainboard/emulation/qemu-i440fx/memory.c

@@ -44,11 +44,3 @@ unsigned long get_top_of_ram(void)
 {
 	return qemu_get_memory_size() * 1024;
 }
-
-#if CONFIG_DYNAMIC_CBMEM
-void *cbmem_top(void)
-{
-	/* Top of cbmem is at lowest usable DRAM address below 4GiB. */
-	return (void *)get_top_of_ram();
-}
-#endif