board: add support for RIoTboard

This is a small development board, based on Freescale IMX6 Solo SoC
(single core ARM Cortex-A9). The board has excellent support in mainline
U-Boot and Linux.

Signed-off-by: Nikolay Dimitrov <picmaster@mail.bg>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>