MIPS: cpu-probe: Fix VTLB/FTLB configuration for R6

MIPS: cpu-probe: Fix VTLB/FTLB configuration for R6

R6 has dropped the MMUExtDef field from the config4 register and it
now returns 0. However, the return value means nothing in that case
and the only supported configuration for R6 is the VTLB+FTLB
(MMUextDef == 3). As a result, rework the code so that the correct
value is set for R6 cores.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/10651/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>