kernel: backport MIPS changes introducing a separate IRQ stack

Prevents crashes when IRQs arrive when the current kernel stack context
already contains deeply nested function calls, e.g. when stacking lots
of network devices on top of each other

Signed-off-by: Felix Fietkau <nbd@nbd.name>

target/linux/brcm47xx/patches-4.4/159-cpu_fixes.patch

@@ -260,7 +260,7 @@
  #endif /* _ASM_R4KCACHE_H */
 --- a/arch/mips/include/asm/stackframe.h
 +++ b/arch/mips/include/asm/stackframe.h
-@@ -358,6 +358,10 @@
+@@ -365,6 +365,10 @@
  		.macro	RESTORE_SP_AND_RET
  		LONG_L	sp, PT_R29(sp)
  		.set	arch=r4000