sim:config: Add configurable mips control at the springbok core

Can change the springbok core executation speed without affecting the
secure and SMC cores' IO.

Springbok core doen't have much IO communication (UART) so it can a
less interactive IO and faster translation process. It is the same
strategy applied by QEMU.

Set the springbok core MIPS default to be 1000 MIPS, 10x faster than the
Renode CPU default.

Change-Id: I63d6f2745b4d7e50ca41bd076b4a83b73c58b59c