cpu/boards: completed GPIO driver for sam3x8e

- adjusted peripheral conf for udoo and arduino-due
- added gpio_init_int()
- added interrupt service routines
- fixed gpio_read()
- added clock enable for gpio_init_in()
Merge pull request #1486 from haukepetersen/fix_sam3x_finish_gpio

cpu/boards: completed GPIO driver for sam3x8e