[rtl,flash_ctrl] Remove PHY_ALERT_CFG CSR

[matutem]

Description

The PHY_ALERT_CFG CSR's outputs are unconnected, and the CSR itself can be removed. This would cause changes in CSR offsets, so it would become an incompatible change requiring bumping up the major version of the IP.

[Razer6]

Couldn't youi create a skip-to and create a hole?

[vogelpi]

To not require bumping the major version but avoid the coverage holes I would rather update the definition of the CSR to always read as 0, and to update the documentation inline with that.