[mlir][ArmSME] Update tile slice layout syntax (#69151)

This patch prefixes tile slice layout with `layout` in the
assemblyFormat:

  - `<vertical>`   -> `layout<vertical>`
  - `<horizontal>` -> `layout<horizontal>`

The reason for this change is the current format doesn't play nicely
with additional optional operands, required to support padding and
masking (#69148), as it becomes ambiguous.

This affects the the following ops:

  - arm_sme.tile_load
  - arm_sme.tile_store
  - arm_sme.load_tile_slice
  - arm_sme.store_tile_slice