rockchip: tinker: configs: use correct mmc instance as boot target device

We are using wrong mmc instance as boot target device now. below Jaehoon Chung's
patch use mmc alias which correct it. That make tinker board can not find mmc
device. So give it correct mmc device instance.

        commit 02ad33aa
        Author: Jaehoon Chung <jh80.chung@samsung.com>
        Date:   Thu Feb 2 13:41:14 2017 +0900

            mmc: mmc-uclass: use the fixed devnum with alias node

Signed-off-by: Eddie Cai <eddie.cai.linux@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>