RISCV: Register clocksource and events correctly

Currently, timer_probe() is called for every cpu and clocksource
is registered multiple times for each cpu which is wrong.

Probe timer only once during init and register the clock source at
that time. Move the clock event registration cpu online notification
callback. Take this opportunity to remove redundant functions as well.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>