Documentation/powerpc: Mention 40x is removed

Commit 732b32da ("powerpc: Remove core support for 40x") removed 40x.

Update documentation accordingly.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/c2d64bebc514ca892a12e51a68821a6317048d3a.1720694954.git.christophe.leroy@csgroup.eu