x86: coral: Add a devicetree node for eMMC

Add a node for this so we can indicate that it is does not require any
ACPI code.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>