This patch extends CLINT emulation to provide rdtime callback for
TCG. This rdtime callback will be called wheneven TIME CSRs are
read in privileged modes.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>