csky: Set regs->usp to kernel sp, when the exception is from kernel

In the past, we didn't care about kernel sp when saving pt_reg. But in some
cases, we still need pt_reg->usp to represent the kernel stack before enter
exception.

For cmpxhg in atomic.S, we need save and restore usp for above.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>

arch/csky/kernel/atomic.S

@@ -17,10 +17,12 @@ ENTRY(csky_cmpxchg)
 	mfcr	a3, epc
 	addi	a3, TRAP0_SIZE
 
-	subi    sp, 8
+	subi    sp, 16
 	stw     a3, (sp, 0)
 	mfcr    a3, epsr
 	stw     a3, (sp, 4)
+	mfcr	a3, usp
+	stw     a3, (sp, 8)
 
 	psrset	ee
 #ifdef CONFIG_CPU_HAS_LDSTEX
@@ -47,7 +49,9 @@ ENTRY(csky_cmpxchg)
 	mtcr	a3, epc
 	ldw     a3, (sp, 4)
 	mtcr	a3, epsr
-	addi	sp, 8
+	ldw     a3, (sp, 8)
+	mtcr	a3, usp
+	addi	sp, 16
 	KSPTOUSP
 	rte
 END(csky_cmpxchg)