MIPS: Add accessor & bit definitions for GlobalNumber

MIPSr6 introduces a GlobalNumber register, which is required when VPs
are implemented (ie. when multi-threading is supported) but otherwise
optional. The register contains sufficient information to uniquely
identify a VP within a system using its cluster number, core number & VP
ID.

In preparation for using this register & its fields, introduce an
accessor macro for it & define its various bits with the typical style
preprocessor macros.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/17007/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>