[WATCHDOG] convert AT91RM9200 watchdog to platform driver

Converted to a platform driver.
Added suspend/resume support - the watchdog is disabled during the
sleep states.

Original patch from David Brownell.

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