Allow detecting whether or not U-Boot was launched through the
recovery mode of the resp. NVIDIA SoC.

Make use of a board specific arch_misc_init() and enable the same via
CONFIG_ARCH_MISC_INIT configuration option.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>