MIPS: Loongson-2F: Flush the branch target history in BTB and RAS

As per chapter 15 "Errata: Issue of Out-of-order in loongson"[1] to work
around the Loongson 2F erratum we need to do:

"When switching from user mode to kernel mode, you should flush the
branch target history such as BTB and RAS."

[1] Chinese version: http://www.loongson.cn/uploadfile/file/200808211
[2] English version of chapter 15:
    http://groups.google.com.hk/group/loongson-dev/msg/e0d2e220958f10a6?dmode=sourceSigned-off-by: NWu Zhangjin <wuzhangjin@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: Shinya Kuribayashi <shinya.kuribayashi@necel.com>
Patchwork: http://patchwork.linux-mips.org/patch/1066/Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/stackframe.h

@@ -121,6 +121,25 @@
 		.endm
 #else
 		.macro	get_saved_sp	/* Uniprocessor variation */
+#ifdef CONFIG_CPU_LOONGSON2F
+		/*
+		 * Clear BTB (branch target buffer), forbid RAS (return address
+		 * stack) to workaround the Out-of-order Issue in Loongson2F
+		 * via its diagnostic register.
+		 */
+		move	k0, ra
+		jal	1f
+		 nop
+1:		jal	1f
+		 nop
+1:		jal	1f
+		 nop
+1:		jal	1f
+		 nop
+1:		move	ra, k0
+		li	k0, 3
+		mtc0	k0, $22
+#endif /* CONFIG_CPU_LOONGSON2F */
 #if defined(CONFIG_32BIT) || defined(KBUILD_64BIT_SYM32)
 		lui	k1, %hi(kernelsp)
 #else