irqchip/aspeed-i2c-ic: Add binding docs for Aspeed I2C Interrupt Controller

Added device tree binding documentation for Aspeed I2C Interrupt
Controller.

Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>