powerpc/machdep: Make machine name const

Machine name in struct machdep_calls should never be modified.

Mark it const.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/6cb9865d916231c38401ba34ad1a98c249fae135.1676711562.git.christophe.leroy@csgroup.eu