MIPS: lantiq: export clk_get_io() for lantiq_wdt.ko

The lantiq WDT driver uses clk_get_io(), which is not exported,
so export it to fix a build error:

ERROR: modpost: "clk_get_io" [drivers/watchdog/lantiq_wdt.ko] undefined!

Fixes: 287e3f3f

 ("MIPS: lantiq: implement support for clkdev api")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: John Crispin <john@phrozen.org>
Cc: linux-mips@vger.kernel.org
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>