MIPS: bcm47xx: Include missing errno.h for ENXIO

Fixes the following build problen on allmodconfig:

arch/mips/bcm47xx/board.c: In function 'bcm47xx_board_detect':
arch/mips/bcm47xx/board.c:291:14: error: 'ENXIO' undeclared
(first use in this function)

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6571/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>