ARM: orion: move watchdog setup to mach-orion5x

The watchdog device node is created in plat-orion/common.c
but depends on the bridge address that is platform specific,
so as a preparation for orion multiplatform support, we
move it out of the common code into orion5x and dove.

At the moment, dove does not use the watchdog, so I'm marking
the function as __maybe_unused for the moment. The compiler
will be able to compile out the device definition this way,
and we can easily add it later.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>