Add support for the built in watchdog in AT32AP700X devices.

Tested on AT32AP7000 and ATSTK1000.

Hardware documentation can be found in the AT32AP7000 datasheet.
Signed-off-by: Hans-Christian Egtvedt <hcegtvedt@atmel.com>
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>