MIPS: Add board_ebase_setup()

Some systems need to relocate the MIPS exception vector base during
trap initialization.  Add a hook to make this possible.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2959/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>