[WATCHDOG] at32ap700x_wdt.c - Add spinlock support

Add spinlock support so that forked children can't
do different io stuff at the same time.

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