powerpc: Add cputime_to_nsecs()

Generic code has a wrapper to implement cputime_to_nsecs() on top of
cputime_to_usecs() but we can easily return the full nanosecond
resolution directly.
Signed-off-by: Anton Blanchard <anton@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200713083601.1103978-1-anton@ozlabs.org

arch/powerpc/include/asm/cputime.h

@@ -36,6 +36,8 @@ static inline unsigned long cputime_to_usecs(const cputime_t ct)
 	return mulhdu((__force u64) ct, __cputime_usec_factor);
 }
 
+#define cputime_to_nsecs(cputime) tb_to_ns((__force u64)cputime)
+
 /*
  * PPC64 uses PACA which is task independent for storing accounting data while
  * PPC32 uses struct thread_info, therefore at task switch the accounting data