genirq: Add chip_[suspend|resume] PM support to irq_chip

Some (admittedly odd) irqchips perform functions that are not directly
related to any of their child IRQ lines, and therefore need to perform
some tasks during suspend/resume regardless of whether there are
any "installed" interrupts for the irqchip. However, the current
generic-chip framework does not call the chip's irq_{suspend,resume}
when there are no interrupts installed (this makes sense, because there
are no irq_data objects for such a call to be made).

More specifically, irq-bcm7120-l2 configures both a forwarding mask
(which affects other top-level GIC IRQs) and a second-level interrupt
mask (for managing its own child interrupts). The former must be
saved/restored on suspend/resume, even when there's nothing to do for
the latter.

This patch adds a new set of suspend/resume hooks to irq_chip_generic,
to help represent *chip* suspend/resume, rather than IRQ suspend/resume.
These callbacks will always be called for an IRQ chip (regardless of the
installed interrupts) and are based on the per-chip irq_chip_generic
struct, rather than the per-IRQ irq_data struct.

The original problem report is described in extra detail here:
http://lkml.kernel.org/g/20150619224123.GL4917@ld-irv-0074

Signed-off-by: Brian Norris <[email protected]>
Tested-by: Florian Fainelli <[email protected]>
Cc: Gregory Fong <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: Kevin Cernekee <[email protected]>
Cc: Jason Cooper <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Thomas Gleixner <[email protected]>

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