hw: timer: ibex_timer: update/add reg address

hw: timer: ibex_timer: update/add reg address

The following changes:
1. Fixes the incorrectly set CTRL register address. As
per [1] https://docs.opentitan.org/hw/ip/rv_timer/doc/#register-table

The CTRL register is @ 0x04.

This was found when attempting to fixup a bug where a timer_interrupt
was not serviced on TockOS-OpenTitan.

2. Adds ALERT_TEST register as documented on [1], adding repective
   switch cases to error handle and later implement functionality.

Signed-off-by: Wilfred Mallawa <wilfred.mallawa@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Tested-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-id: 20220111071025.4169189-2-alistair.francis@opensource.wdc.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>