dt-bindings/interrupt-controller: pdc: Describe PDC device binding

Add device binding documentation for the PDC Interrupt controller on
QCOM SoC's like the SDM845. The interrupt-controller can be used to
sense edge low interrupts and wakeup interrupts when the GIC is
non-operational.

Cc: devicetree@vger.kernel.org
Signed-off-by: Archana Sathyakumar <asathyak@codeaurora.org>
Signed-off-by: Lina Iyer <ilina@codeaurora.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>