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>
4 files changed