hw/riscv: Provide rdtime callback for TCG in CLINT emulation

hw/riscv: Provide rdtime callback for TCG in CLINT emulation

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>