watchdog: MIPS: lantiq: implement OF support and minor fixes

Add support for OF. We also apply the following small fixes
* reduce boiler plate by using devm_request_and_ioremap
* sane error path for the clock
* move LTQ_RST_CAUSE_WDTRST to a soc specific header file
* add a message to show that the driver loaded

Signed-off-by: John Crispin <blogic@openwrt.org>
Acked-by: Wim Van Sebroeck <wim@iguana.be>
Cc: linux-watchdog@vger.kernel.org
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3810/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>