board: turris: Do not cache Atsha device in BSS

Atsha device is used prior relocation and at this early stage BSS does not
have to be ready yet. So do not cache Atsha device in BSS.

Fixes support for other Turris routers.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>