powerpc/boot: Remove all 40x platforms from boot

Remove 40x platforms from the boot directory.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240628121201.130802-2-mpe@ellerman.id.au