RISC-V: time.c: Add ACPI support for time_init()

On ACPI based platforms, timer related information is
available in RHCT. Add ACPI based probe support to the
timer initialization.

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>