[rv_core_ibex] Add u-mode performance counters and mcounteren

[SamuelRiedel]

• Update Ibex to contain the u-mode performance counter aliases
• Ibex also implements the mcounteren CSR to control which u-mode counters are accessible
• The mcounteren CSR can be locked through an input signal to prevent modifying it even in m-mode
• Add a mcounteren_writable register in rv_core_ibex driving the above signal. This register is lockable with the mcounteren_writable_regwen register.

ToDo:

• Add sofware test to ensure the newly added registers are writable, lockable, and correctly lock the mcounteren register in Ibex
• Use consistent MuBi encoding between Ibex and OT.
• Update Ibex once Add mcounteren register ibex#2403 is merged

[gautschimi]

Looks all good.

changes in this PR are approved. But the vendor-in commit needs to be updated once the ibex PR has been merged.

edit: commit messages could be a bit longer:)

edit: and the CI jobs need to be fixed