Disable MIPS64 ISAs for MIPS32 targets

Currently you can select MIPS64 ISAs, like mips64 and mips64r2, for
MIPS32 targets. This is incorrect, so we disable the possibility to do
that.

Reviewed-by: Markos Chandras <markos.chandras@imgtec.com>
Signed-off-by: Vicente Olivert Riera <vincent.riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>