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

arch/powerpc/platforms/Kconfig.cputype

@@ -137,7 +137,7 @@ config GENERIC_CPU
 
 config POWERPC_CPU
 	bool "Generic 32 bits powerpc"
-	depends on PPC32 && !PPC_8xx && !PPC_85xx
+	depends on PPC32 && !PPC_8xx && !PPC_85xx && !40x
 
 config CELL_CPU
 	bool "Cell Broadband Engine"
@@ -179,6 +179,10 @@ config E6500_CPU
 	bool "Freescale e6500"
 	depends on PPC64 && E500
 
+config 405_CPU
+	bool "40x family"
+	depends on 40x
+
 config 860_CPU
 	bool "8xx family"
 	depends on PPC_8xx
@@ -223,6 +227,7 @@ config TARGET_CPU
 	default "power7" if POWER7_CPU
 	default "power8" if POWER8_CPU
 	default "power9" if POWER9_CPU
+	default "405" if 405_CPU
 	default "860" if 860_CPU
 	default "e300c2" if E300C2_CPU
 	default "e300c3" if E300C3_CPU