MIPS: Fix typo BIG_ENDIAN to CPU_BIG_ENDIAN

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: eed0eabd12ef0 ("MIPS: generic: Introduce generic DT-based board support")
Signed-off-by: Corentin Labbe <[email protected]>
Reviewed-by: James Hogan <[email protected]>
Cc: Ralf Baechle <[email protected]>
Cc: [email protected]
Cc: <[email protected]> # 4.9+
Patchwork: https://patchwork.linux-mips.org/patch/18495/
[[email protected]: Clean up commit message]
Signed-off-by: James Hogan <[email protected]>