Add support for the Ibex timer. This is used with the RISC-V
mtime/mtimecmp similar to the SiFive CLINT.

We currently don't support changing the prescale or the timervalue.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-id: 716fdea2244515ce86a2c46fe69467d013c03147.1624001156.git.alistair.francis@wdc.com