MIPS: lantiq: add clk_round_rate()

This adds a basic implementation of clk_round_rate()
The clk_round_rate() function is called by multiple drivers and
subsystems now and the lantiq clk driver is supposed to export this,
but doesn't do so, this causes linking problems like this one:
ERROR: "clk_round_rate" [drivers/media/v4l2-core/videodev.ko] undefined!

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: John Crispin <blogic@openwrt.org>
Cc: <stable@vger.kernel.org> # 4.1+
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/11358/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>