clocksource/drivers/riscv: Add required checks during clock source init
Currently, clocksource registration happens for an invalid cpu for
non-smp kernels. This lead to kernel panic as cpu hotplug registration
will fail for those cpus. Moreover, riscv_hartid_to_cpuid can return
errors now.
Do not proceed if hartid or cpuid is invalid. Take this opportunity to
print appropriate error strings for different failure cases.