[WATCHDOG] at32ap700x_wdt: add support for boot status and add fix for silicon errata

This patch enables the watchdog to read out the reset cause after a boot and
provide this to the user.

The driver will now also return -EIO if probed when booting from a watchdog
reset. This is due to a silicon errata in the AT32AP700x devices.

Detailed description and work-arounds can be found in the errata section of the
datasheet avilable from
http://www.atmel.com/dyn/products/datasheets.asp?family_id=682

Signed-off-by: Hans-Christian Egtvedt <hcegtvedt@atmel.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>