gpio: mcp23s08: Add irq functionality for i2c chips

This adds interrupt functionality for i2c chips to the driver.
They can act as a interrupt-controller and generate interrupts, if
the inputs change.
This is tested with a mcp23017 chip on an arm based platform.

v3:
- be a bit more clear that the irq functionality is also available
  on spi versions of the chips, but the linux driver does not support
  this yet

v2:
- some more word about irq-mirror property in binding doc
- use of_read_bool instead of of_find_property for
  "interrupt-contrller" and "irq-mirror"
- cache the "interrupt-controller" for remove function
- do set the irq-mirror bit only if device is marked as
  interrupt-controller
- do create the irq mapping and setup of irq_desc of all possible
  interrupts in probe path instead of in gpio_to_irq
- mark gpios as in use as interrupts in irq in irq_startup and
  unlock it in irq_shutdown
- rename virq to child_irq
- remove dev argument from mcp23s08_irq_setup function
- move gpiochip_add before mcp23s08_irq_setup in probe path

Signed-off-by: Lars Poeschel <poeschel@lemonage.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/Kconfig

@@ -722,6 +722,7 @@ config GPIO_MCP23S08
 	  SPI/I2C driver for Microchip MCP23S08/MCP23S17/MCP23008/MCP23017
 	  I/O expanders.
 	  This provides a GPIO interface supporting inputs and outputs.
+	  The I2C versions of the chips can be used as interrupt-controller.
 
 config GPIO_MC33880
 	tristate "Freescale MC33880 high-side/low-side switch"