[mlir][quant] Bump up the MaxStorageBits from 32 to 64.

[dansuh17]

64-bit storage type for quantized type is often used for certain settings like on-device or when dealing with models that are quality-sensitive. For example, a TFLite micro kernel supports 64-bit quantized types for 16x8 quantized operations.

Currently the Quant dialect allows up to 32 bit storage bits: MaxStorageBits. Bump this limit up to 64.

[dansuh17]

(first time contributing directly on github - how do I assign myself to this issue? I'm planning on working on this)

[bviyer]

(first time contributing directly on github - how do I assign myself to this issue? I'm planning on working on this)

I did it for you.

[dansuh17]

Thanks!