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/cpu-probe.c

@@ -20,6 +20,7 @@
 
 #include <asm/bugs.h>
 #include <asm/cpu.h>
+#include <asm/cpu-type.h>
 #include <asm/fpu.h>
 #include <asm/mipsregs.h>
 #include <asm/watch.h>
@@ -55,7 +56,7 @@ static inline void check_errata(void)
 {
 	struct cpuinfo_mips *c = &current_cpu_data;
 
-	switch (c->cputype) {
+	switch (current_cpu_type()) {
 	case CPU_34K:
 		/*
 		 * Erratum "RPS May Cause Incorrect Instruction Execution"