MIPS: SEAD-3: Only include in 32 bit kernels by default

The MIPS SEAD-3 development board has only ever been used with 32 bit
CPUs, so including support for it in 64 bit kernels is wasteful since
those kernels will never run on a SEAD-3.

Specify a requirement in the SEAD-3 board config fragment that ensures
the board support is only included in 32 bit kernels, by checking that
CONFIG_32BIT=y.
Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16944/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/configs/generic/board-sead-3.config

+# require CONFIG_32BIT=y
+
 CONFIG_LEGACY_BOARD_SEAD3=y
 
 CONFIG_AUXDISPLAY=y