watchdog: davinci: rename platform driver to davinci-wdt

As we switch to use the watchdog core which permits more than one
active watchdog in the system, rename platform driver to
"davinci-wdt" to be identifiable.

Acked-by: Wim Van Sebroeck <wim@iguana.be>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>