powerpc: Remove core support for 40x

Now that 40x platforms have gone, remove support
for 40x in the core of powerpc arch.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240628121201.130802-4-mpe@ellerman.id.au