clocksource: riscv_timer: split boot and secondary cpu timer init

Since clocksource is system-wide and timers are local to each cpu,
split the initialization routines to match this layout.

The boot cpu initializes the clocksource and its own timer, while
the secondary cpus initialize only their timers since clocksource
is already set up.

Signed-off-by: Dmitriy Cherkasov <dmitriy@oss-tech.org>