e500: ppce500_init_mpic() return device instead of IRQ array

Actual number of interrupt pins isn't known
in ppce500_init_mpic() so a hardcoded number
was used, which causes a crash with older openpic.

Instead, return the DeviceState* and change ppce500_init()
to call qdev_get_gpio_in() to get only the irq pins
which are needed.

Signed-off-by: Michael Davidsaver <mdavidsaver@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
