powerpc/8xx: fix invalid register expression in head_8xx.S

New binutils generate the following warning

  AS      arch/powerpc/kernel/head_8xx.o
arch/powerpc/kernel/head_8xx.S: Assembler messages:
arch/powerpc/kernel/head_8xx.S:916: Warning: invalid register expression

This patch fixes it.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/head_8xx.S

@@ -913,7 +913,7 @@ start_here:
 	tovirt(r6,r6)
 	lis	r5, abatron_pteptrs@h
 	ori	r5, r5, abatron_pteptrs@l
-	stw	r5, 0xf0(r0)	/* Must match your Abatron config file */
+	stw	r5, 0xf0(0)	/* Must match your Abatron config file */
 	tophys(r5,r5)
 	stw	r6, 0(r5)