[WATCHDOG] Blackfin on-chip watchdog driver

This patch implements the driver necessary use the Analog Devices
Blackfin processor's on-chip watchdog controller, supports
BF53[123]/BF53[467]/BF54[2489]/BF561.

Signed-off-by: Mike Frysinger <michael.frysinger@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>