ARM: imx: Add bmode support for iMX7

Add the basic differentiation between i.MX6 and i.MX7 into the bmode
command, the mechanism really works almost the same on both platforms.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: NXP i.MX U-Boot Team <uboot-imx@nxp.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>