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/kernel/time.c

@@ -24,6 +24,7 @@
 #include <linux/export.h>
 
 #include <asm/cpu-features.h>
+#include <asm/cpu-type.h>
 #include <asm/div64.h>
 #include <asm/smtc_ipi.h>
 #include <asm/time.h>