powerpc/40x: Remove 40x platforms.

40x platforms have been orphaned for many years.

Remove them.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240628121201.130802-1-mpe@ellerman.id.au