This patch adds the support for new PIO controller found on some
at91sam SOCs.
- more peripheral multiplexing
- more features to configure on a PIO (pull-down, Schmitt trigger, debouncer)
- support for several IRQ triggering features (type and polarity)

Support for those new features are retrieved from the device tree
compatibility string.

Debugfs at91_gpio file is updated to monitor configuration.
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>