MIPS: Provide nice way to access boot CPU's data.

boot_cpu_data is used the same as current_cpu_data but returns the CPU
data for CPU 0.  This means it doesn't have to use smp_processor_id()
thus no need to disable preemption.
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/cpu-info.h

@@ -84,6 +84,7 @@ struct cpuinfo_mips {
 extern struct cpuinfo_mips cpu_data[];
 #define current_cpu_data cpu_data[smp_processor_id()]
 #define raw_current_cpu_data cpu_data[raw_smp_processor_id()]
+#define boot_cpu_data cpu_data[0]
 
 extern void cpu_probe(void);
 extern void cpu_report(void);