arm: imx: stop sata on boot

Ideally, the Linux kernel should get the hardware in its most
untouched state. For the most part, U-Boot does not reset the various
subsystems it touches before boot, and usually Linux deals with it, but
on some boards (cm_fx6) the Linux kernel fails to detect the ssd
correctly if sata is used by U-Boot.

Power off sata on OS boot so that Linux will have a clean state to work
with.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Stefano Babic <sbabic@denx.de>