MIPS: Octeon: board_type_to_string: return NULL for unsupported board

Return NULL for unsupported board.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: David Daney <ddaney.cavm@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/12581/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>