powerpc: drop hard_reset_now() and poweroff_now() declaration

Those function have never existed. Drop their declaration.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/edcdd72a36495d25213c0256c8022367458e0d19.1596716418.git.christophe.leroy@csgroup.eu