ARM: imx: Add support for switching primary/secondary boot mode to bmode

ARM: imx: Add support for switching primary/secondary boot mode to bmode

The i.MX6/i.MX7 is capable of booting a secondary "redundant" system
image in case the primary one is corrupted. The user can force this
boot mode as well by explicitly setting SRC GPR10 bit 30. This can be
potentially useful when upgrading the bootloader itself. Expose this
functionality to 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>