MIPS: Kconfig: Only allow 32-bit microMIPS builds

Only allow 32-bit microMIPS builds, we're not ready yet for 64-bit
microMIPS support.

QEMU does have support for the 64-bit microMIPS ISA and with minor
tweaks it is possible to have a 64-bit processor emulated there that
runs microMIPS code, so despite the lack of actual 64-bit microMIPS
hardware there is a way to run 64-bit microMIPS Linux, but it can all be
considered early development and we are not there yet.  Userland tools
are lacking too, e.g. GCC produces bad code:

{standard input}: Assembler messages:
{standard input}:380: Warning: wrong size instruction in a 16-bit branch delay slot

And our build fails early on, so disable the configuration, for the sake
of automatic random config checkers if nothing else.  Whoever needs to
experiment with 64-bit microMIPS support can revert this change easily.

Signed-off-by: Maciej W. Rozycki <macro@codesourcery.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/8481/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>