ARM: i.MX: fimware: Add pre-built BL31 ATF blob ...

Running Linux on i.MX8M requires us to use and properly set up a ARM
Trusted Firmware blob. For the sake of convenience, add a pre-built
blob to our firmware tree, so it can be used by individual boards.

This blob was built using Buildroot:

 - commit: 6b02b82
 - defconig: freescale_imx8mqevk_defconfig

The resulting file of interest is output/images/bl31.bin

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>