pinmux_dev: Add the pinmux_dev driver for Atmel SAM3X

This introduces the pinmux_dev driver for the Atmel SAM3X.

This driver implements what used to be the pinmux driver API, which
applications could use to modify the function of pins during runtime.

That functionality is now protected under the CONFIG_PINMUX_DEV option,
which should only be set during the early enabling of a new board, as
there is risk of damage to the board when misused.

Change-Id: I3aa00505d2771b53c41fe687c3e5230e804756be
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>