hw/riscv/opentitan: add aon_timer base unimpl

hw/riscv/opentitan: add aon_timer base unimpl

Adds the updated `aon_timer` base as an unimplemented device. This is
used by TockOS, patch ensures the guest doesn't hit load faults.

Signed-off-by: Wilfred Mallawa <wilfred.mallawa@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20221025043335.339815-3-wilfred.mallawa@opensource.wdc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>