watchdog: orion: Move the register ioremap'ing to its own function

Follow-up patches will extend the registers ioremap and request
to handle SoC-specific quirks on the RSTOUT register. Therefore,
in order to keep the code readable, this commit introduces a special
function for this.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Tested-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>