powerpc/mpic: constify copied structure

The mpic_ipi_chip and mpic_irq_ht_chip structures are only copied
into other structures, so make them const.

The opportunity for this change was found using Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1577864614-5543-10-git-send-email-Julia.Lawall@inria.fr