[WATCHDOG] ar7_wdt: convert to become a platform driver

This patch converts the ar7_wdt driver to become
a platform driver. The AR7 SoC specific identification
and base register calculation is performed by the board
code, therefore we no longer need to have access to
ar7_chip_id. We also remove the reboot notifier code to
use the platform shutdown method as Wim suggested.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>