clocksource: timer-riscv: Don't enable/disable timer interrupt

Currently, we enable/disable timer interrupt at runtime to start/stop
timer events. This makes timer interrupt state go out-of-sync with
the Linux interrupt subsystem.

To address the above issue, we can stop a per-HART timer interrupt by
setting U64_MAX in timecmp CSR (or sbi_set_timer()) at time handling
timer interrupt.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>