gpiolib to_irq() feature

gpiolib to_irq() feature

Add a new gpiolib mechanism:  gpio_chip instances can provide
mappings between their (input) GPIOs and any associated IRQs.
This makes it easier for platforms to support IRQs that are
provided by board-specific external chips instead of as part
of their core (such as SOC-integrated GPIOs).

Also update the irq_to_gpio() description, saying to avoid it
becuase it's not always supported.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>