hw/riscv: OpenTitan: Connect the mtime and mtimecmp timer

hw/riscv: OpenTitan: Connect the mtime and mtimecmp timer

Connect the Ibex timer to the OpenTitan machine. The timer can trigger
the RISC-V MIE interrupt as well as a custom device interrupt.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-id: 5e7f4e9b4537f863bcb8db1264b840b56ef2a929.1624001156.git.alistair.francis@wdc.com