ARM: dts: imx7d-sdb: add eMMC5.0 support

imx7d-sdb board has a eMMC5.0 on usdhc3. This eMMC support HS400.
This patch add usdhc3 support for HS400

Signed-off-by: Haibo Chen <haibo.chen@freescale.com>
Acked-by: Dong Aisheng <aisheng.dong@freescale.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>