MIPS: Kconfig: Don't allow both microMIPS and SmartMIPS to be selected.

microMIPS and SmartMIPS can't be used together. This fixes the
following build problem:

Warning: the 32-bit microMIPS architecture does not support the `smartmips' extension
arch/mips/kernel/entry.S:90: Error: unrecognized opcode `mtlhx $24'
[...]
arch/mips/kernel/entry.S:109: Error: unrecognized opcode `mtlhx $24'

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/7421/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>