Implement support for i.MX7 Sabre board

Implement support for i.MX7 Sabre board

Implement code needed to set up emulation of MCIMX7SABRE board from
NXP. For more info about the HW see:

https://www.nxp.com/support/developer-resources/hardware-development-tools/sabre-development-system/sabre-board-for-smart-devices-based-on-the-i.mx-7dual-applications-processors:MCIMX7SABRE

Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Philippe Mathieu-Daudé <f4bug@amsat.org>
Cc: Marcel Apfelbaum <marcel.apfelbaum@zoho.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: qemu-devel@nongnu.org
Cc: qemu-arm@nongnu.org
Cc: yurovsky@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>