MIPS: Fix some missing CONFIG_CPU_MIPSR6 #ifdefs

Commit be0c37c9 (MIPS: Rearrange PTE bits into fixed positions.)
defines fixed PTE bits for MIPS R2. Then, commit d7b63141


(MIPS: pgtable-bits: Fix XPA damage to R6 definitions.) adds the MIPS
R6 definitions in the same way as MIPS R2. But some R6 #ifdefs in the
later commit are missing, so in this patch I fix that.

Signed-off-by: Huacai Chen <chenhc@lemote.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Cc: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: Fuxin Zhang <zhangfx@lemote.com>
Cc: Zhangjin Wu <wuzhangjin@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: stable@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12164/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>