MIPS: Optimize current_cpu_type() for better code.

 o Move current_cpu_type() to a separate header file
 o #ifdefing on supported CPU types lets modern GCC know that certain
   code in callers may be discarded ideally turning current_cpu_type() into
   a function returning a constant.
 o Use current_cpu_type() rather than direct access to struct cpuinfo_mips.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Cc: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5833/
diff --git a/arch/mips/mm/dma-default.c b/arch/mips/mm/dma-default.c
index 664e5236..f25a7e9 100644
--- a/arch/mips/mm/dma-default.c
+++ b/arch/mips/mm/dma-default.c
@@ -18,6 +18,7 @@
 #include <linux/highmem.h>
 
 #include <asm/cache.h>
+#include <asm/cpu-type.h>
 #include <asm/io.h>
 
 #include <dma-coherence.h>