ARM: zImage: __armv3_mpu_cache_flush: respect should-be-zero specification

Probably the register content for cache operations is "don't care" in
practice, but as r1 is explicitly zeroed, use that one.
Acked-by: Eric Miao <eric.miao@canonical.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

arch/arm/boot/compressed/head.S

@@ -994,7 +994,7 @@ no_cache_id:
 __armv3_mmu_cache_flush:
 __armv3_mpu_cache_flush:
 		mov	r1, #0
-		mcr	p15, 0, r0, c7, c0, 0	@ invalidate whole cache v3
+		mcr	p15, 0, r1, c7, c0, 0	@ invalidate whole cache v3
 		mov	pc, lr
 
 /*