[WATCHDOG] Atmel AT91RM9200 rename.

The new Atmel AT91SAM9261 and AT91SAM9260 processors use a different
internal watchdog peripheral.  This watchdog driver is therefore
AT91RM9200-specific.

This patch renames at91_wdt.c to at91rm9200_wdt.c, and changes the name
of the configuration option.

Signed-off-by: Andrew Victor <andrew@sanpeople.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>