Connect KelvinCPU with e2e nexus simulation

Need to add some nop API to make springbok/kelvin module share the same
interface in .resc. Also remove the GDBServer for cpu2 (kelvin core
debugging should be done in kelvin_sim)

Retire the temporary kelvin renode platform file to merge back to
`kelvin_ml_core.repl`

Change-Id: Ic9e104f5f2fef6230d77accfdb2a7271a1072a0f