powerpc/8xx: Move CPM interrupt controller into a dedicated file

CPM interrupt controller is quite standalone. Move it into a
dedicated file. It will help for next step which will change
it to a platform driver.

This is pure code move, checkpatch report is ignored at this point,
except one parenthesis alignment which would remain at the end of
the series. All other points fly away with following patches.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/d3a7dc832d905bed14b35d83410cdb69a7ba20e8.1649226186.git.christophe.leroy@csgroup.eu