powerpc/64s/exception: denorm handler use standard scratch save macro

Although the 0x1500 interrupt only applies to bare metal, it is better
to just use the standard macro for scratch save.

Runtime code path remains unchanged (due to instruction patching).
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/exceptions-64s.S

@@ -1925,7 +1925,7 @@ EXC_REAL_NONE(0x1400, 0x100)
 EXC_VIRT_NONE(0x5400, 0x100)
 
 EXC_REAL_BEGIN(denorm_exception_hv, 0x1500, 0x100)
-	mtspr	SPRN_SPRG_HSCRATCH0,r13
+	SET_SCRATCH0(r13)
 	EXCEPTION_PROLOG_0 PACA_EXGEN
 	EXCEPTION_PROLOG_1 EXC_HV, PACA_EXGEN, 0, 0x1500, 0, 0, 0