MIPS: NI 169445: Only include in 32r2el kernels

The NI 169445 board uses a little endian MIPS32r2 CPU, and therefore
including board support in kernels that are unable to run on such a CPU
is pointless.

Specify requirements in the board config fragment that cause the NI
169445 board support to only be included in generic kernels that target
little endian MIPS32r2 CPUs.

For example, NI 169445 support will be included when configuring using
32r2el_defconfig but not when using 64r6_defconfig.
Signed-off-by: NPaul Burton <paul.burton@imgtec.com>
Acked-by: NNathan Sullivan <nathan.sullivan@ni.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16945/Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

arch/mips/configs/generic/board-ni169445.config

+# require CONFIG_CPU_MIPS32_R2=y
+# require CONFIG_CPU_LITTLE_ENDIAN=y
+
 CONFIG_FIT_IMAGE_FDT_NI169445=y
 
 CONFIG_SERIAL_8250=y