powerpc/8xx: Remove mpc8xx_pics_init()

mpc8xx_pics_init() is now only a trampoline to
mpc8xx_pic_init().

Remove mpc8xx_pics_init() and use mpc8xx_pic_init()
directly.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/9c55a698adb5ba3b7b77023170fcaf0acb5d2d81.1649226186.git.christophe.leroy@csgroup.eu