mpic_irq_set_wake return -ENXIO for non FSL MPIC and sets IRQF_NO_SUSPEND
flag for FSL ones. enable_irq_wake already returns -ENXIO if irq_set_wak
is not implemented. Also there's no need to set the IRQF_NO_SUSPEND flag
as it doesn't guarantee wakeup for that interrupt.

This patch removes the redundant mpic_irq_set_wake and sets the
IRQCHIP_SKIP_SET_WAKE for only FSL MPIC.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Hongtao Jia <hongtao.jia@freescale.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>