RISC-V: Remove per cpu clocksource

There is only one clocksource in RISC-V. The boot cpu initializes
that clocksource. No need to keep a percpu data structure.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>