MIPS: DEC: Only select the R4k clock event/source on R4k systems

R3k systems have no R4k timer so there's no point in pulling code that's
going to be dead.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6704/


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