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

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>