Make async callback ice_smem_read/write_async parameter

[alastairpatrick]

Make smem module thead safe. Allow RTOS to block during async serial memory operation. Allow async callback to immediately chain another async op.

[alastairpatrick]

Locking isn't an RTOS feature. I've removed all mention of RTOS from the comments to avoid confusion. The purpose of locking is to provide software based arbitration of the serial memory bus between both cores (but not between ARM cores and FPGA).

I think it would be possible for a higher level module to do this, or at least it would be with a few simpler modifications to ice_smem and am open to that. It would mean that ice_smem would generally be unsafe to use on both cores at the same time.