Most i.MX6SL infrastructure is already covered in barebox by general i.MX6
support. Missing infrastructure provided in separate commits are
* SoC type detection
* Clock infrastructure

Add the missing fsl,imx6sl-mmdc, so it will not be catched by fsl,imx6q-mmdc
and the remaining bits and pieces to provide barebox i.MX6SL SoC support.

Signed-off-by: Alexander Kurz <akurz@blala.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>