genirq: Allow the irqchip state of an IRQ to be save/restored

There is a number of cases where a kernel subsystem may want to
introspect the state of an interrupt at the irqchip level:

- When a peripheral is shared between virtual machines,
  its interrupt state becomes part of the guest's state,
  and must be switched accordingly. KVM on arm/arm64 requires
  this for its guest-visible timer
- Some GPIO controllers seem to require peeking into the
  interrupt controller they are connected to to report
  their internal state

This seem to be a pattern that is common enough for the core code
to try and support this without too many horrible hacks. Introduce
a pair of accessors (irq_get_irqchip_state/irq_set_irqchip_state)
to retrieve the bits that can be of interest to another subsystem:
pending, active, and masked.

- irq_get_irqchip_state returns the state of the interrupt according
  to a parameter set to IRQCHIP_STATE_PENDING, IRQCHIP_STATE_ACTIVE,
  IRQCHIP_STATE_MASKED or IRQCHIP_STATE_LINE_LEVEL.
- irq_set_irqchip_state similarly sets the state of the interrupt.

Signed-off-by: Marc Zyngier <[email protected]>
Reviewed-by: Bjorn Andersson <[email protected]>
Tested-by: Bjorn Andersson <[email protected]>
Cc: [email protected]
Cc: Abhijeet Dharmapurikar <[email protected]>
Cc: Stephen Boyd <[email protected]>
Cc: Phong Vo <[email protected]>
Cc: Linus Walleij <[email protected]>
Cc: Tin Huynh <[email protected]>
Cc: Y Vo <[email protected]>
Cc: Toan Le <[email protected]>
Cc: Bjorn Andersson <[email protected]>
Cc: Jason Cooper <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Thomas Gleixner <[email protected]>

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