serial: MIPS: lantiq: add clk_enable() call to driver

Enable the clock if one is present when setting up the console.

Signed-off-by: Thomas Langer <thomas.langer@lantiq.com>
Acked-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>