[prim] Formality encounters syntax errors

### Description

**Formality reports a syntax error when reading this line**

https://github.com/lowRISC/opentitan/blob/f49474bc89c3ce8a99536329a1e16bf22936da37/hw/ip/prim/rtl/prim_assert_sec_cm.svh#L115

I believe the reason is that `!rst_ni` is illegal.

I’ve looked into why only this line reports an error. Other assertions use `ASSERT_DEFAULT_RST`, so I think this is a simple oversight. Other tools happen to support this syntax and do not throw errors.

https://github.com/lowRISC/opentitan/blob/f49474bc89c3ce8a99536329a1e16bf22936da37/hw/ip/prim/rtl/prim_assert.sv#L18

https://github.com/lowRISC/opentitan/blob/f49474bc89c3ce8a99536329a1e16bf22936da37/hw/ip/prim/rtl/prim_assert.sv#L119

Give a description of the problem, listing similar issues and if able:
- Assign the issue to someone familiar with it.
- Add appropriate labels.

Start at hw/ip/prim/rtl/prim_assert_sec_cm.svh line 115 and compare the assertion patterns with hw/ip/prim/rtl/prim_assert.sv lines 18 and 119. Use Formality to check the affected assertion and confirm that the syntax error is resolved without introducing failures in the related assertions.

Written by the indexing model from the issue text.