target/mips: Add MXU decoding engine

Add MXU decoding engine: add handlers for all instruction pools,
and main decode handler. The handlers, for now, for the purpose
of this patch, contain only sceleton in the form of a single
switch statement.

Reviewed-by: Stefan Markovic <smarkovic@wavecomp.com>
Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>