MIPS: Add P6600 PRID & cpu_type_enum values

MIPS: Add P6600 PRID & cpu_type_enum values

Define the processor ID for the P6600 core and add a value to the enum
cpu_type_enum for the core.

[ralf@linux-mips.org: Fix merge conflict.]

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Joshua Kinard <kumba@gentoo.org>
Cc: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12342/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>