MIPS: kernel: entry.S: Add MIPS R6 related definitions

The instruction hazard barrier in the form of:

jr.hb	ra
nop

is valid on MIPS R6 as well.
Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>

arch/mips/kernel/entry.S

@@ -158,7 +158,8 @@ syscall_exit_work:
 	jal	syscall_trace_leave
 	b	resume_userspace
 
-#if defined(CONFIG_CPU_MIPSR2) || defined(CONFIG_MIPS_MT)
+#if defined(CONFIG_CPU_MIPSR2) || defined(CONFIG_CPU_MIPSR6) || \
+    defined(CONFIG_MIPS_MT)
 
 /*
  * MIPS32R2 Instruction Hazard Barrier - must be called
@@ -171,4 +172,4 @@ LEAF(mips_ihb)
 	nop
 	END(mips_ihb)
 
-#endif /* CONFIG_CPU_MIPSR2 or CONFIG_MIPS_MT */
+#endif /* CONFIG_CPU_MIPSR2 or CONFIG_CPU_MIPSR6 or CONFIG_MIPS_MT */