[rv_timer] Align interrupt behavior with privileged spec

[a-will]

Description

The interrupt currently is cleared for any write to mtimecmp, but this was a misinterpretation of the spec. Ensure the interrupt is asserted as long as mtime is greater than or equal to mtimecmp, with no glitching.

While implementing this change, take advantage of the expanded interrupt capabilities, and convert the rv_timer interrupt to the Status type to avoid awkward latching behavior.

From https://github.com/lowRISC/opentitan/issues/18400#issuecomment-1590249373

[a-will]

Without this fix, software will likely need to ensure the interrupts are not enabled while modifying mtimecmp (to avoid spurious interrupts while performing the multiple writes to update the value).

[msfschaffner]

CC @GregAC @rswarbrick

We may want to align this with the spec for PROD.

[andreaskurth]

Assigning @GregAC to please take a look and propose a resolution -- feel free to delegate the implementation ofc

[vogelpi]

Moving to Backlog.