SOLVED
Dear Opticron,
I would like to thank you for all advices. The BSP is fine. Sorry I had doubt about it.
The problem with the context switch arised with SMP on and number of active harts set to 1, too. It was because the context switch function uses AMO and Polarfire e51 hart does not support AMO. So, I have do decide if really I want and I need to modify beaglevfire BSP to run on e51 hart but it represents my problem.
Thank you again and bye.