irq-imgpdc: add ImgTec PDC irqchip driver

Add irqchip driver for the ImgTec PowerDown Controller (PDC) as found in
the TZ1090. The PDC has a number of general system wakeup (SysWake)
interrupts (which would for example be connected to a power button or an
external peripheral), and a number of peripheral interrupts which can
also wake the system but are connected straight to specific low-power
peripherals (such as RTC or Infrared). It has a single interrupt output
for SysWakes, and individual interrupt outputs for each peripheral.

The driver demuxes the SysWake interrupt line, and passes the peripheral
interrupts straight through. It also handles the set_wake interrupt
operation to enable/disable the appropriate wake event bits.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Grant Likely <grant.likely@linaro.org>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Rob Landley <rob@landley.net>
Cc: linux-metag@vger.kernel.org
Cc: linux-doc@vger.kernel.o...