MIPS: sead3: remove unused cpu_khz variable

This variable seems to have been copied from Malta when SEAD3 support
was introduced, but is likewise unused. Remove it.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Reviewed-by: Markos Chandras <markos.chandras@imgtec.com>
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6172/