MIPS: ralink: Add missing clk_round_rate().

As we dont use the common clock api yet we need to add this stub to allow
building drivers that use the API.

Signed-off-by: John Crispin <john@phrozen.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/14900/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>