MIPS: add support for P6600 cores

-march=p6600 is not yet supported in GCC upstream, so disabling all
versions when selecting this core.

Note that P6600 implies a MIPS R6 CPU, and some GCC versions are already
disabled for R6, so we don't need to disable those ones for P6600 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: Peter Korsgaard <peter@korsgaard.com>