MIPS: add support for M6201 cores

-march=m6201 is not yet supported in GCC upstream, so disabling all
versions when selecting this core.

Note that M6201 implies a MIPS R6 CPU, and some GCC versions are already
disabled for R6, so we don't need to disable those ones for M6201 as
well.

The external Codescape IMG GNU Linux Toolchain has support for this
core.

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>