MIPS: Octeon: Add support for accessing the boot vector.

Used by the Octeon watchdog driver to get the address of the
firmware boot vector.

Signed-off-by: Steven J. Hill <steven.hill@cavium.com>
Acked-by: David Daney <david.daney@cavium.com>
Cc: linux-mips@linux-mips.org
Cc: linux-watchdog@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/17206/


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