[SPIR-V][Doc] add draft SPIR-V extension for bfloat16 atomics

[bashbaug]

Adds the proposed SPV_INTEL_shader_atomic_bfloat16 extension specification, which extends the floating-point atomic support in SPIR-V to support floating-point atomic addition, minimum, and maximum on 16-bit bfloat16 floating-point numbers in memory.

[bashbaug]

We may want to consider broadening this extension to support 16-bit atomics more generally vs. just bfloat16 atomics. Here's what this might look like:

• Add a capability for 16-bit integer atomics, perhaps Int16Atomics, similar to the Int64Atomics capability for 64-bit integer atomics. We could slice this more finely if we would like and add capabilities for specific 16-bit atomic operations.
• Figure out a more appropriate name for the extension, since it wouldn't just be for bfloat16 atomics anymore.

Just like in this current draft for bfloat16 atomics, I don't think we would need any new instructions, and would just support 16-bit types for existing atomic instructions.