ARM: imx: Add support for reading out the primary/secondary bmode to MX7

Implement the 'getprisec' subcommand of 'bmode' command for i.MX7 by
reading out the SRC GPR10 bit 30. This bit is either set by the BootROM
if it switched to the secondary copy due to primary copy being corrupted
OR it can be overridden by the user.

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>
Reviewed-by: Stefano Babic <sbabic@denx.de>