ARM: imx: Add support for the primary/secondary bmode to MX53

Implement the 'getprisec' subcommand of 'bmode' command for i.MX53 and
also the primary/secondary bootmode switching.

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>