[reggen] Consider adding an ALERT_TEST_REGWEN register

@thommythomaso is already working on this.

Since Jun 22, 2026.

### Description

@cfrantz pointed out that it would be better to lock down the ALERT_TEST CSR of all HWIP blocks. This CSR can be used for alert testing. Every HWIP has it and it's auto-generated by the regtool and inserted directly after the also auto-generated interrupt registers at the beginning of an IP's address space. This makes it cumbersome to lock access to the ALERT_TEST CSR for production software.

To simplify that, the following ideas came to mind:
1. Add an ALERT_TEST_REGWEN register. This is low effort and complexity but touches every HWIP. Unless the register is added at the end of the address map, a major version increase for every HWIP is triggered.
2. Re-arrange the address map of every HWIP to move the ALERT_TEST register to a a different address in a space which is a power-of-2 segment apart from the beginning of the HWIPs address space. The power-of-2 separation enables efficient locking via ePMP.