powerpc/8xx: Move mpc8xx_pic.c from sysdev to platform/8xx

mpc8xx_pic.c is dedicated to the 8xx, so move it to platform/8xx

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>