MIPS: mark R4K clockevent device with CLOCK_EVT_FEAT_PERCPU

The CLOCK_EVT_FEAT_PERCPU flag indicates that a clockevent device is
only configurable by the CPU for which it is registered, and thus cannot
be used as the tick broadcast device. That property is true of the R4K
timer, which is inaccessible from other cores.

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