Watchdog: allow orion_wdt to be built for Dove

The watchdog infrastructure in Dove is no different from that in
Orion5x or Kirkwood, so let's enable it for Dove.  The only things
missing are a few register settings in Dove's bridge-regs.h.

Rather than duplicating the same register bit masks for the RSTOUTn_MASK
and BRIDGE_CAUSE registers, move the definitions into the watchdog
driver itself.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>