submodule riscv-pk not updated

[hsqStephenZhang]

I founded that's riscv-pk is not the latest one. And one bug has been fixed in riscv-pk's master branch.
They replace '__clear_cache(0, 0)' with 'asm volatile ("fence.i")'. It was merged in riscv-software-src/riscv-pk#240,
So I wish the submodule riscv-pk could be update.

[aswaterman]

This repository (riscv-tools) is no longer maintained.