watchdog: at91rm9200: use the system timer syscon

Use a syscon regmap to access the system timer registers.

Also, rename the driver atmel_st_watchdog to stop conflicting with
at91sam9_wdt.c

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>