clocksource/timer-riscv: Add ACPI support

Initialize the timer driver based on RHCT table on ACPI based
platforms.

Currently, ACPI doesn't support a flag to indicate that the
timer interrupt can wake up the cpu irrespective of its
power state. It will be added in future update.

Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>