[ARM] Orion/Kirkwood: rename orion5x_wdt to orion_wdt

The Orion watchdog driver is also used on Kirkwood.

Convention is to use orion5x for stuff specific to 88F5xxx Orion chips
and simply "orion" for shared stuff across SoCs including Kirkwood.

Signed-off-by: Nicolas Pitre <nico@marvell.com>