MIPS: Detect MIPSr6 Virtual Processor support

MIPSr6 introduces support for "Virtual Processors", which are
conceptually similar to VPEs from the now-deprecated MT ASE. Detect
whether the system supports VPs using the VP bit in Config5, adding
cpu_has_vp for use by later patches.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: Maciej W. Rozycki <macro@imgtec.com>
Cc: Joshua Kinard <kumba@gentoo.org>
Cc: Steven J. Hill <sjhill@realitydiluted.com>
Cc: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12327/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>