x86: Make sure that the LPC is active before SDRAM init

Some boards need to access GPIOs to determine which SDRAM is fitted to the
board, for example chromebook_link. Probe this device (if it exists) to
make sure that this works as expected.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>