MIPS: kernel: cpu-bugs64: Do not check R6 cores for existing 64-bit bugs

The current HW bugs checked in cpu-bugs64, do not apply to R6 cores
and they cause compilation problems due to removed <R6 instructions,
so do not check for them for the time being.

Reviewed-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>