powerpc/405: Fix build failure with GCC 12 (unrecognized opcode: `wrteei')

Building ppc40x_defconfig leads to following error

  CC      arch/powerpc/kernel/process.o
{standard input}: Assembler messages:
{standard input}:626: Error: unrecognized opcode: `wrteei'

Add -mcpu=405 by default.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/d344a42c99061cfe10a28e00de4e31a1363f4251.1657549153.git.christophe.leroy@csgroup.eu