bsp/tms570: implemented and tested initialization of Cortex-R performance counters.

The code is written as BSP specific now but it should work for all
Cortex-A and R based CPUs and can be moved to ARM generic place in future.

StackOverflow suggested sequences of writes to the registers required
to start counters is used.

http://stackoverflow.com/questions/3247373/how-to-measure-program-execution-time-in-arm-cortex-a8-processor