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/

arch/mips/mm/tlbex.c

@@ -30,6 +30,7 @@
 #include <linux/cache.h>
 
 #include <asm/cacheflush.h>
+#include <asm/cpu-type.h>
 #include <asm/pgtable.h>
 #include <asm/war.h>
 #include <asm/uasm.h>