MIPS: Fix typo BIG_ENDIAN to CPU_BIG_ENDIAN

MIPS_GENERIC selects some options conditional on BIG_ENDIAN which does
not exist.

Replace BIG_ENDIAN with CPU_BIG_ENDIAN which is the correct kconfig
name. Note that BMIPS_GENERIC does the same which confirms that this
patch is needed.

Fixes: eed0eabd

 ("MIPS: generic: Introduce generic DT-based board support")
Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Reviewed-by: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: <stable@vger.kernel.org> # 4.9+
Patchwork: https://patchwork.linux-mips.org/patch/18495/
[jhogan@kernel.org: Clean up commit message]
Signed-off-by: James Hogan <jhogan@kernel.org>