powerpc: Allow CPU selection of G4/74xx variant

GCC supports -mcpu=G4

This patch gives the opportunity to select ALTIVEC for this variant.
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/Kconfig.cputype

@@ -153,6 +153,11 @@ config E300C3_CPU
 	bool "e300c3 (831x)"
 	depends on PPC_BOOK3S_32
 
+config G4_CPU
+	bool "G4 (74xx)"
+	depends on PPC_BOOK3S_32
+	select ALTIVEC
+
 endchoice
 
 config TARGET_CPU_BOOL
@@ -171,6 +176,7 @@ config TARGET_CPU
 	default "860" if 860_CPU
 	default "e300c2" if E300C2_CPU
 	default "e300c3" if E300C3_CPU
+	default "G4" if G4_CPU
 
 config PPC_BOOK3S
 	def_bool y