powerpc: Fix boot wrapper memcmp() called with zero length argument

I noticed, when trying to use, e.g.,
node = find_node_by_prop_value(prev, "booleanprop", "", 0))
to search for all nodes with a certain boolean property, that memcmp()
returns garbage when comparing zero bytes. It should return zero.

Signed-off-by: Benjamin Herrenschmidt <[e-mail]>

arch/powerpc/boot/string.S

···
 	.globl	memcmp
 memcmp:
 	cmpwi	0,r5,0
-	blelr
+	ble	2f
 	mtctr	r5
 	addi	r6,r3,-1
 	addi	r4,r4,-1
···
 	lbzu	r0,1(r4)
 	subf.	r3,r0,r3
 	bdnzt	2,1b
+	blr
+2:	li	r3,0
 	blr