Add minimal code to support NXP i.MX7 SABRESD board. Tested to have
working SD card and first Ethernet port as well as being able to boot
upstream Linux kernel (4.12+).

Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>