xtensa: fix a7 clobbering in coprocessor context load/store

Fast coprocessor exception handler saves a3..a6, but coprocessor context
load/store code uses a4..a7 as temporaries, potentially clobbering a7.
'Potentially' because coprocessor state load/store macros may not use
all four temporary registers (and neither FPU nor HiFi macros do).
Use a3..a6 as intended.

Cc: stable@vger.kernel.org
Fixes: c658eac6 ("[XTENSA] Add support for configurable registers and coprocessors")
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/kernel/coprocessor.S

@@ -29,7 +29,7 @@
 	.if XTENSA_HAVE_COPROCESSOR(x);					\
 		.align 4;						\
 	.Lsave_cp_regs_cp##x:						\
-		xchal_cp##x##_store a2 a4 a5 a6 a7;			\
+		xchal_cp##x##_store a2 a3 a4 a5 a6;			\
 		jx	a0;						\
 	.endif
 
@@ -46,7 +46,7 @@
 	.if XTENSA_HAVE_COPROCESSOR(x);					\
 		.align 4;						\
 	.Lload_cp_regs_cp##x:						\
-		xchal_cp##x##_load a2 a4 a5 a6 a7;			\
+		xchal_cp##x##_load a2 a3 a4 a5 a6;			\
 		jx	a0;						\
 	.endif