target-mips: add mips32r6-generic CPU definition

Define a new CPU definition supporting MIPS32 Release 6 ISA and
microMIPS32 Release 6 ISA.

Signed-off-by: Yongbok Kim <yongbok.kim@imgtec.com>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>