powerpc/40x: Fix assembler warning about r0

The assembler says:
arch/powerpc/kernel/head_40x.S:623: Warning: invalid register expression

It's objecting to the use of r0 as the RA argument. That's because
when RA = 0 the literal value 0 is used, rather than the content of
r0, making the use of r0 in the source potentially confusing.

Fix it to use a literal 0, the generated code is identical.

Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/powerpc/kernel/head_40x.S

···
 	ori	r6, r6, swapper_pg_dir@l
 	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)