MIPS: allow R4K clockevent device to function regardless of GIC

Having the GIC clockevent driver compiled should not prevent the R4K
timer clockevent driver from functioning. One will be selected as the
CPU local timer based upon their priorities and the other may simply be
unused or in the case of the GIC timer may be used as the tick broadcast
device.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>