xlnx-ep108: Fix minimum RAM check

The minimum RAM check logic for the Xiilnx EP108 was off by one,
which caused a false positive. Correct the logic to only print
warnings when the RAM is below 0x8000000.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: fba8112ca7b01efd72553332b8045ecf107b7662.1448021100.git.alistair.francis@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
